Periodic Classification of Elements Class 10 Science Exam Questions

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Class 10 Science Exam Questions Periodic Classification Of Elements

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Very Short Answer:

Question:  State the Modern periodic law of classification of elements.
Answer : Modern periodic law states that the physical and chemical properties of elements are the periodic function of their atomic numbers.

Question: What is the valency of silicon with atomic number 14? 
Answer : Atomic number of silicon = 14
Electronic configuration = 2, 8, 4
As silicon (Si) contains four electrons in its outermost shell, its valency will be four.

Question. In the Modern Periodic Table, Calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21, and 38. Which of these have physical and chemical properties resembling calcium ?
Answer : The element with atomic number 12 and 38 has same chemical properties as that of Calcium because they have same number of valence electrons i.e., 2 electrons.

Question. The atomic radii of first group elements are given below : 
Group-I element       Atomic Radii (pm)
Na                                       86
K                                        231
Rb                                      244
Cs                                       282
State the reason behind the observed trend in the above elements. 
Answer : The above trend is observed because the atomic radii increases down the group, because new shells are being added as we go down the group. This increases the distance between the outermost electrons and the nucleus so that the atomic size increases in spite of the increase in nuclear charge.

Question. How would the tendency to lose electrons change as we go from left to right across a period of the periodic table ?
Answer : On moving from left to right in a period, the tendency of atoms to lose electrons decreases.

Question. The electronic configuration of an element is 2, 8, 4. State its :
(a) group and period in the Modern Periodic Table.
(b) name and write its one physical property.
Answer : (a) Si = 2, 8, 4
(i) It means that it belongs to 3rd period and 14th group.
(ii) The name of element is Silicon.
(b) It is a metalloid. (i.e., element that has properties of botsh metals and non-metals).

Question. Define electropositivity.
Answer : Electropositivity is the measure of the ability of elements (mainly metals) to donate their valence electrons to form positive ions. The elements that can easily lose electrons to form positive ions are called electropositive elements. For example : Metals such as aluminium, sodium etc.

Question. Write the formulae of chlorides of Eka-silicon and Eka-aluminium.
Answer : GeCl4, GaCl3.

Question. Why do you think the noble gases are placed in a separate group ?
Answer : Noble gases are placed in a separate group because these are inert elements. They have properties which are different from all other elements.

Question. What were the criteria used by Mendeleev in creating his periodic table ?
                        OR 
State Mendeleev’s Periodic Law.
Answer : Mendeleev’s periodic table was based on atomic masses and similarity in formula of hydrides and oxides of elements. According to him, the properties of elements are a periodic function of their atomic masses.

Question. The atomic number of an element is 17. To which group and period does this element belong ? Determine its valency.
Answer : Element with atomic number 17 belongs to Group 17 and 3 period and its valency is 1.

Question. In the modern periodic table a zig-zag line separates metals from non-metals. What are these elements called and why ?
Answer : These are known as metalloids because they have some properties of metals and some properties of non-metals.

Question. State Modern Periodic Law of classification of elements.
                                             OR
How many vertical columns are there in the Modern Periodic Table and what are they called ?
                                                                       OR
State Modern Periodic Law. What is number of groups and periods in the Modern Periodic Table ?
Answer : “Properties of elements are a periodic function of their atomic number”. There are 18 groups and 7 periods are there in the Modern Periodic Table.

Question. Give the number of a group in which metallic, metalloid and non-metallic, all three types of elements, are present.
Answer : Group 14.

Question. Write the name, symbol and electronic configuration of an element X whose atomic number is 11.
Answer : The element whose atomic number is 11 is sodium. Its symbol is Na.
Electronic configuration – 2, 8, 1
Valency 1.

Question:  List any two properties of the elements belonging to the first group of the Modern Periodic Table.
Answer : Two properties of the elements belonging to the first group :
(i) As the elements belong to group 1, so they have one electron in their outermost shell hence, valency of these elements is one.
(ii) Alkali metals (group 1 elements) are electropositive in nature.

Question: Write the formula used to determine the maximum number of electrons which a shell in an atom can accommodate. 
Answer : The maximum number of electrons that can be accommodated in a shell, is given by the formula 2ⁿ², where ‘n’ is the number of the shell.

Question:  Out of the three elements P, Q and R having atomic numbers 11, 17 and 19 respectively, which two elements will show similar properties and why? 
Answer : Atomic number of P = 11
Electronic configuration of P = 2, 8, 1
Electronic configuration of Q(17) = 2, 8, 7
and for R(19) = 2, 8, 8, 1
Thus, from electronic configurations of P and R, it is observed that they belong to group 1 as both have one valence electron and have valency equal
to 1. thus, P and R will have similar properties.

Question. F, Cl and Br are the elements each having seven valence electrons. Which of these (a) has the largest atomic radius, (b) is most reactive? 
Answer:
a. Br has the largest atomic size because it has 4 shells.
b. F is the most reactive v it can give electrons most easily due to its smallest size.

Question. List any two properties of elements belonging to the first group of modem periodic table.
Answer:
a. They have 1 valence electron.
b. Their valency is equal to 1.

Question. What is a metalloid? Name any two of them.
Answer: Metalloids are those elements which resembles both with metals as well as non-metals e.g., Boron and Silicon.

Question. Write the atomic numbers of two elements X and Y having electronic configurations 2, 8, 2 and 2, 8, 6, respectively.
Answer:
X = 2 + 8 + 2 = 12
Y = 2 + 8 + 6 = 16

Question. An element M has atomic number 12.
a. Write its electronic configuration.
b. State the group to which M belongs to.
c. Is M a metal or a non-metal?
d. Write the formula of its oxide.
Answer:
a. 2, 8, 2,
b. Group 2,
c. Metal,
d. MO is formula of its oxide.

Question. Name any two elements which were discovered later after the formation of Mendeleev’s periodic table.
Answer: Gallium and Germanium.

Question. The atomic number of three elements A, B and C are 12, 18 and 20 respectively. State giving reason, which two elements will show similar properties.
Answer: A(12): 2, 8, 2, B(18): 2, 8,8, C(20): 2, 8, 8, 2 A and C belong to the same group because they have same number of valence electrons.

Question. What is the valency of silicon with atomic number 14?
Answer: Si(2, 8, 4), the number of valence electrons in Si is 4. Therefore, its valency is 4.

Question. Would you place the two isotopes Cl-35 and Cl-37 in different slots because of their different atomic masses or in the same slot because their chemical properties are same? 
Answer: They will be placed in the same slot because they have similar chemical properties due to same number of valence electrons.

Question. Why are H, Li, Na and K placed in group-1?
Answer: It is because they have one valence electron.

Question:  How does electronic configuration of atoms change in a period with increase in atomic number?
Answer : One electron each is added successively to the same valence shell as we move from left to right in the period. Due to this increase in valence electrons, the electronic configuration of atoms changes with increase in atomic number.

 Short Answer: I

Question:  An element ‘X’ belongs to 3^rd period and group 17 of the periodic table. State its
(i) electronic configuration, (ii) valency.
Justify your answer with reason.
Answer: As element X belongs to group 17, it will have 7 electrons in its outermost shell. Moreover, X belongs to period number 3 so, it will have 3 shells.
(i) Electronic configuration of X = 2, 8, 7
(ii) Valency of element X = 8 – (Number of valence electrons) = 8 – 7 = 1

Question:  How does the valency of elements vary (i) in going down a group, and (ii) in going from left to right in a period of the periodic table?
Answer: (i) When we go down the group the valency of elements remains same.
(ii) When we move along the period from left to right, the valency of elements first increases and then decreases.

Question:  In the Modern Periodic Table, the element calcium (atomic number = 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these elements has physical and chemical properties resembling those of calcium and why? 
Answer: From the given data, the electronic configuration of different elements can be written as :
Calcium (20) = 2, 8, 8, 2
Element with atomic number 12 = 2, 8, 2
Element with atomic number 19 = 2, 8, 8, 1
Element with atomic number 21 = 2, 8, 8, 3
Element with atomic number 38 = 2, 8, 18, 8, 2
It can be easily seen that elements with atomic numbers 12 and 38 have two electrons in their outermost shell thus, they belong to same group as that of calcium. So, they will show the physical and chemical properties resembling those of calcium.

Question: In the periodic table, how does the tendency of atoms to loose electrons change on going from 
(i) left to right across a period?
(ii) top to bottom in a group?
Answer: (i) Tendency of atoms to loose electrons decreases from left to right in a period due to increase in effective nuclear charge.
(ii) Tendency of atoms to loose electrons increases down the group due to increase in atomic radii.

Question. The atomic number of an element is 12.
a. Write its electronic configuration and determine its valency.
b. Is it more reactive or less reactive than Ca(20)?
c. Is it a metal or a non-metal?
d. Write the formula of its oxide.
Answer:  a. Mg(12): 2, 8, 2. Its valency is equal to 2.
b. It is less reactive than Ca.
c. It is a metal.
d. MgO

Question. An element X’ is placed in 13th group and 3rd period of Modern Periodic Table. Answer the following stating reason for your answer.
a. Write the electronic configuration of the element X’.
b. Write the formula of the compound formed when element X’ reacts with another element ‘Y’, having atomic number 17.
c. Will oxide of this element be acidic or basic?
Answer: 
X is Aluminium (13); Y(17) 2, 8, 7
a. 2, 8, 3

c. It will be amphoteric i.e., it is acidic as well as basic.

Question. The electronic configuration of 4 elements A, B, C and D are given as under:
A-2, 1; B-2, 2; C-2, 8, 2; D-2
a. Which amongst them belongs to the same group?
b. Which amongst them belong to the same period?
c. Which amongst them is an inert element? 
Answer: 
a. B and C belong to the same group, 2nd group; Be and Mg.
b. A and B belong to the same period, 2nd period; Li and Be.
c. D is the inert; It is Helium (He).

Question. (a) State modern periodic law.
(b) How many groups and periods are present in the modern periodic table?
(c) State how the problem of placing (a) hydrogen,
(b) isotopes of an element has been solved in this periodic table.
Answer: 
a. Properties of elements are a periodic function of their atomic numbers’.
b. There are 18 groups and 7 periods.
c. (i) Hydrogen is placed along with group 1 as well as halogens because it resembles with both of them.
(ii) Isotopes do not need a separate place as they have the same atomic number.

Question. Given below are some elements of the Modern Periodic Table. Atomic number of the element is given in the parentheses:
A(4), B(9), C(14), D(19), E(20)
a. Select the element that has only one electron in the outermost shell. Also write the electronic configuration of this element.
b. Which two elements amongst these belong to the same group? Give reason for your answer.
c. Which two elements amongst these belong to the same period? Which among the two has bigger atomic radius?
Answer: 
a. D(19): 2,8,8,1 has 1 valence electron.
b. A(4): 2, 2 and E(20): 2, 8, 8, 2 belong to same group because they have the same number of valence electrons.
c. A(2, 2), B(2, 7) belong to the same period. A has bigger atomic radius than B.

Question. Two elements X and Y have atomic 12 and 17 respectively.
a. Write the electronic configuration of both these elements.
b. Which type of bond will they form?
c. Write the formula of the compound formed by their combination (in terms of X and Y).
Answer: 
a. X: 2, 8, 2; Y: 2, 8,7
b. They will form ionic bond.

XY₂ is the formula of the compound.

Question. An element ‘P’ (atomic number 20) reacts with an element ‘Q’ (atomic number 17) to form a compound.
Answer the following questions giving reason:
Write the position of ‘P’ and ‘Q’ in the Modem Periodic Table and the molecular formula of the compound formed when ‘P* reacts with ‘Q’.
Answer: 
P(20): 2, 9, 9, 2; Q(17): 2, 8, 7 ‘P’ belongs to group 2 and 4th period.
‘Q’ belongs to group 17 and 3rd period. 98
PQ2 is the molecular formula of the compound formed.

Question. The atomic number of these elements are given below:

Arrange this elements in increasing order of their atomic numbers. Give reason for your answer.
Answer: 
B(5), C(6), N(7), 0(8) is the increasing order of their atomic numbers.
It is because when we move along a period, atomic radii decreases due to increase in effective nuclear charge due to increase in number of protons and electrons continuously.

Question. An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a compound.
a. Write the position of these elements in the modern periodic table.
b. Write the formula of the compound formed.
Answer: 
a. X: 2, 8, 7 belongs to 3rd period, group 17, because it has 3 shells and 7 valence electrons.
Y: 2, 8, 8, 2 belongs 4th period, group 2, because
it has 4 shells and 2 valence electrons. b. YX2, is the formula of the compound, because Y has valency 2, X has valency 1.

Question. The atomic number of K and Ca is 19 and 20 respectively and they belong to same period.
a. Which amongst them would have smaller atomic size?
b. Which one would be more electro¬positive?
c. To which group would each one belong to?
Answer: 
a. Ca has smaller atomic size.
b. K is more electropositive.
c. K belongs to group 1, Ca belongs to group 2.

Question. Na, Mg, A1 are the elements having one, two and three valence electrons respectively. Which of these elements (a) has longest atomic radius, (b) is least
reactive?
Answer: 
a. Na has the largest atomic radius due to least effective nuclear charge due to less forces of attraction between 11 protons and 11 electrons.
b. A1 is the least reactive element due to small its size and least tendency to lose electrons.

Question. Two elements X and Y have atomic number 11 and 16 respectively.
a. Write the electronic configuration of both.
b. Write the formula of the compound formed by their combination (in terms of X and Y).
Answer: 
a. X(11): 2, 8, 1; Y(16): 2, 8, 6
b. X2Y is the formula of compound formed.

Question. Four elements P, Q, R and S have atomic numbers 12, 13, 14 and 15 respectively. Answer the following questions giving reasons:
a. What is the valency of Q?
b. Classify these elements as metals and non-metals.
c. Which of these elements will form the most basic oxide?
Answer: 
a. Q(2, 8, 3) has valency = 3
b. P, Q are metals, R and S are non¬metals.
c. P will form the most basic oxide.

Question. The atomic number an element is 17. Predict:
a. its valency.
b. whether it is a metal or a non-metal?
c. its relative size with respect to other members of the group.
Answer:  2, 8, 7 is the electronic configuration.
a. Its valency is equal to 1 because it can gain 1 electron to become stable.
b. Non-metals because it can gain an electron to form anion.
c. It is bigger in size than F (1st element) but smaller than all other members because it has 3 shells, F has 2 shells, others have more than 3 shells.

Question. Na/Mg and A1 are the elements having one, two and three valence electrons respectively. Which of these elements (a) has the largest atomic radius, (b) is least reactive? Justify you answer stating reason for each.
Answer: 
Na has largest atomic radii due to 11 protons and 11 electrons and least effective nuclear charge. Na is most reactive because it can lose an electron easily due to its large size and least effective nuclear charge.

Question: How does the electronic configuration of an atom of an element relate to its position in the Modern Periodic Table? Explain with one example. 
Answer: Electronic configuration of an element decides its position in Modern periodic table.
Lets take an example of sodium (Na). Atomic number of sodium = 11 Thus, electronic configuration of Na = 2, 8, 1 As Na contains 1 electron in its outermost shell, it belongs to group 1. Sodium contains 3 shells so, it belongs to period number 3. Thus, we can conclude that Group number = Number of valence electrons (When valence electrons are 1 and 2) and group number = 10 + valence electrons
(When valence electrons are 3 and above) Period number = Number of shells in which electrons arefilled.

Question:Elements magnesium and oxygen respectively belong to group 2 and group 16 of the Modern Periodic Table. If the atomic numbers of magnesium and oxygen are 12 and 8 respectively, draw their electronic configuration and show the process of formation of their compound by transfer of electrons.
Answer: Atomic number of magnesium = 12
∴ Electronic configuration = 2, 8, 2
Valency of magnesium (Mg) = 2
Similarly, for oxygen (O) atomic number = 8
Electronic configuration = 2, 6
Valency of oxygen = 8 – 6 = 2
Formation of their compound will be as follows :

Question: What is meant by periodicity of properties of elements? Why are the properties of elements placed in the same group of the periodic table similar? 
Answer: When elements are arranged in increasing order of their atomic numbers, elements with similar chemical properties are repeated at definite intervals.This is known as periodicity of properties of elements. Elements placed in the same group of the periodic table have similar properties because they have same number of outermost electrons and hence, show same valency. us, they all will form similar type of compounds.

Short Answers Type Questions

Question: How it can be proved that the basic structure of the Modern Periodic Table is based on the electronic configuration of atoms of different elements ?
Answer : Modern periodic law states that the physical and chemical properties of an element are the periodic function of the atomic number of that element.
Electronic configuration of the elements play an important role in the placement of element in the modern periodic table. The valence shell electron of an element decides its position in a particular group or period for example : if the configuration of an element is 2, 1 it means that the Li = 2, 1 It belongs to the 2nd period and 1st group.

Question: Na, Mg and Al are the elements of the 3rd period of the Modern Periodic Table having group number 1, 2 and 13 respectively. Which one of these elements has the (a) highest valency, (b) largest atomic radius and (c) maximum chemical reactivity ? Justify your answer stating the reason for each.
Answer :
Sodium        (Na), At. number 11,      2, 8, 1
Magnesium  (Mg), At. number 12,     2, 8, 2
Aluminium   (Al), At. number 13,        2, 8, 3
(a) The element having the highest valency is Al, as it has 3 valence electrons.
(b) The element with the largest atomic radius is Na as left to right atomic radius decreases.
(c) The element with maximum chemical reactivity is Na as metallic character decreases left to right.

Question: Can the following groups of elements be classified as Dobereiner’s triad :
(a) Na, Si, Cl (b) Be, Mg, Ca
Atomic mass of Be-9; Na-23, Mg-24, Si-28, Cl-35, Ca-40, Justify your answer in each case.
Answer : (a) Na, Si, Cl cannot be classified as Dobereiner’s triad because here the elements do not belong to the same group and have different electronic configuration.
Na– 2, 8, 1; Si – 2, 8, 4 and Cl – 2, 8, 7
(b) Be, Mg, Ca are the elements of Dobereiner‘s triad because the mass of Mg is the arithmetic mean of the other two elements i.e., Be and

Question: Write the number of periods and groups in the Modern Periodic Table. How does the metallic character of elements vary on moving (a) from left to right in a period and (b) down a group ? Give reason to justify your answer.
Answer : In the Modern Periodic Table, there are 18 vertical columns known as Groups and 7 horizontal rows known as Periods.
Metallic character : It is defined as the tendency of an atom to lose electrons.
(a) Across the period i.e., from left to right, metallic character decreases.
(b) Down the group i.e., from top to bottom, metallic character increases.
Reason : Across the period, the effective nuclear charge increases, thus decreasing its atomic radius. This favours the electronegativity and therefore the tendency to lose electrons is low. This accounts for the decrease in the metallic character. As we move down the group, the number of shells keep on increasing and therefore the atomic size increases and electronegativity decreases.
This enhances the ability to lose electrons and therefore the metallic character increases.

Question: What is periodicity in properties of elements with reference to the Modern Periodic Table ? Why do all the elements of the same group have similar properties ? How does the tendency of elements to gain electrons change as we move from left to right in a period ? State the reason of this change. 
Answer : The occurrence of the elements with similar properties after certain regular intervals when they are arranged in increasing order of atomic number is called periodicity.
The periodic repetition of the properties is due to the recurrence of similar valence shell configuration after regular interval.
The elements in a group have same valence electrons thus similar chemical properties. In a period, tendency to gain electrons increases from left to right. This is due to increase in effective nuclear change and decrease in atomic radius.

Question: An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a divalent halide.
(a) Where in the periodic table are elements X and Y placed ?
(b) Classify X and Y as metal (s), non-metal (s) or metalloid (s).
(c) What will be the nature of oxide of element Y ?
Identify the nature of bonding in the compound formed.
(d) Draw the electron dot structure of the divalent halide.
Answer : (a) X belongs to Group 17 and 3^rd period Y belongs to Group 2 and 4^th period.
(b) X — Non-metal and Y — Metal.
(c) Basic oxide; Ionic bonding.

Question: Use Mendeleev’s periodic table to predict the formulae for the oxides of the following elements : K, C, Al, Si, Ba. 
Answer :
K belongs to group 1. Therefore, the oxide will be K₂O.
C belongs to group 4. Therefore, the oxide will be CO₂.
Al belongs to group 3. Therefore, the oxide will be Al₂O₃.
Si belongs to group 4. Therefore, the oxide will be SiO₂.
Ba belongs to group 2. Therefore, the oxide will be BaO.

Question: Based on the group valency of elements write the molecular formula of the following compounds giving justification for each :
(a) Oxide of first group elements.
(b) Halide of the elements of group thirteen, and
(c) Compound formed when an element, A of group 2 combines with an element, B of group seventeen.
Answer : (a) A₂O, for example :
Sodium is a group one element (Na) so its configuration is 2, 8, 1 and its valency is 1.
Oxide has a valency of 2.
So, their formula would be

Formula for sodium oxide is Na₂O.
(b) AX₃ for example :
Halide is any halogen. Group 13 means for example, we take aluminium (Al) its configuration is 2, 8, 3, valency of Al is 3, valency of Cl is 1.
So,

Formula would be AlCl₃ (Aluminium chloride)
(c) AB₂ for example :
Element of group 2 for example Mg 2, 8, 2 its valency would be 2.
Element of group 17 would be Cl 2, 8, 7 valency would be 1.

Question: Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice ? 
Answer : Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements and they belong to same group.

Question: An element X which is a yellow solid at room temperature shows catenation and allotropy. X forms two oxides which are also formed during the thermal decomposition of ferrous sulphate crystals and are the major air pollutants.
(a) Identify the element X.
(b) Write the electronic configuration of X.
(c) Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals.
(d) What would be the nature (acidic or basic) of oxides formed ?
(e) Locate the position of the element in the modern periodic table.
Answer :
(a) Element X is sulphur (atomic no. 16).
(b) K, L, M
2 8 6

Question: An element P (atomic number 20) reacts with an element Q (atomic number 17) to form a compound.
Answer the following questions giving reason : Write the position of P and Q in the Modern Periodic Table and the molecular formula of the compound formed when P reacts with Q.
Answer : P = 20 : 2, 8, 8, 2
Q = 17 : 2, 8, 7
P = Period 4 and Group 2
Q = Period 3 and Group 17
Hence, formula of the compound formed between P and Q is PQ₂.

Question: Give reasons for the following :
(a) Noble gases do not react with other elements.
(b) Atom is electrically neutral but still it has a tendency to form an ion.
Answer : (a) Elements react in order to complete its octet. Noble gases have complete octet. Therefore, they do not react with other elements.
(b) The elements have valence electrons. Metals have less valence electrons and give out the valence electrons to form cations to complete the outermost shell and attain nearest noble gas configuration. Likewise, non metals attain electrons to form anions to complete the outermost shell and attain nearest noble gas configuration.

Question: An element has same number of electrons in the first and the fourth shell as well as in the second and the third shell.
(a) Write down the electronic configuration of the element.
(b) Write down group number and period to which it belongs.
(c) What is the valency of the element ?
Answer : (a) The electronic configuration of the element is 2, 8, 8, 2. The element is calcium.
(b) Period 4 and group 2.
(c) Valency of element is 2.

Question: (a) What property do all elements in the same column of the periodic table as boron have in common ?
(b) What property do all elements in the same column of the periodic table as fluorine have in common ? 
Answer : (a) All the elements in the same column as boron have the same number of valence electrons i.e., 3. Hence, they all have valency equal to 3.
(b) All the elements in the same column as fluorine have the same number of valence electrons i.e., 7.
Hence, they all have valency equal to 1.

Question: (a) An element X has mass number 40 and contains 21 neutrons in its atom. To which group of the periodic table does it belong ?
(b) The element X forms a compound X₂Y. Suggest an element that Y might be and give reasons for your choice.
Answer : (a) Group 1 (2, 8, 8, 1).
(b) Oxygen (X is monovalent so Y has to be divalent to form the compound X₂Y).

Question: An element ‘M’ has atomic number 12.
(a) Write its electronic configuration and valency.
(b) Is ‘M’ a metal or a non-metal ? Give reason in support of your answer.
(c) Write the formula and nature (acidic / basic) of the oxide of M.
Answer : (a) Electronic configuration of element ‘M’ is 2, 8, 2 and valency is 2.
(b) M is a metal because there are two electrons in its outermost shell and it easily loses the electron to form a positive ion.
(c) The formula of the oxide of M is MO and it is basic in nature.

Question: Compare and contrast the arrangement of elements in Mendeleev’s Periodic Table and the Modern Periodic Table.
Answer :

Question: Consider the following elements (atomic numbers are given in parenthesis)
Ca(20) ; K(19) ; F(9) ; Be(4)
(a) Select :
(i) the element having one electron in the outermost shell
(ii) two elements of the same group. Write the number of this group.
(b) Write the formula of the compound formed by the union of Ca(20) and the element X(2, 8, 7).
Answer : (a) (i) K
(ii) Be and Ca, Group-2
(b) CaX₂

Question: The atomic number of an element is 20.
(a) Write its electronic configuration and determine its valency.
(b) Is it a metal or a non-metal ?
(c) Write the formula of its chloride.
(d) Is it more reactive or less reactive than Mg (atomic number 12) ? Give reason for your answer.
Answer : (a) Electonic configuration of X(20) is 2, 8, 8, 2
Valence electrons-2
Hence, valency is 2.
(b) It is a metal.
(c) XCl₂.
(d) It is more reactive than Mg as reactivity increases down the group since Mg is in III Period and X₂₀
(Ca) is in IV period.

Question: How does the atomic radius of the elements change on going
(a) from left to right in a period, and
(b) down a group in the Modern Periodic Table ?
Give reason in support of your answer.
Answer : (a) Atomic radius decreases from left to right in a period because Nuclear charge increases which tends to pull the electrons closer to the nucleus.
(b) Atomic radius increases down a group because the number of shells increases on going down the group.

Question: Write the electronic configurations of two elements P (atomic number 17) and Q (atomic number 19) and determine their group numbers and period numbers in the Modern Periodic Table. 
Answer : Electronic configuration of ‘P’ : 2,8,7
Group number : 17
Period number : 3
Electronic configuration of Q : 2,8,8,1
Group number : 1
Period number : 4

Question: The atomic number of an element M is 19.
(a) Write electronic configuration and valency of this element.
(b) Is ‘M’ a metal or a non-metal ? Give reason in support of your answer.
(c) Write the formula and nature (acidic/basic) of the oxide of M.
Answer : (a) Electronic configuration of element ‘M’ is 2, 8, 8, 1 and Valency is 1.
(b) M is a metal because it has 1 valence electron which it easily loses to form a positive ion.
(c) The formula of the oxide of M is M₂O, and it is basic in nature.

Question: How does the tendency of the elements to lose electrons change in the Modern Periodic Table in (a) a group, (b) a period and why ?
Answer : (a) Increases down a group because at each succeeding element down a group the number of shells increases, so the distance of the valence shell from the nucleus increases, the effective nuclear force of attraction decreases on the last shell electrons decreases ,so it becomes easy for the atom to loose electrons.
(b) Decreases in a period left to right, because as the effective nuclear charge on the valence shell electron increases, the attraction between the valence electron and nucleus increases, so it becomes difficult to lose electrons.

Question: The atomic number of an element is 19.
(a) Write the electronic configuration of this element and determine (i) the valency of this element and (ii) whether this element is a metal or a nonmetal.
(b) Write the formula of the oxide of this element.
(c) Is this element more reactive or less reactive than Na (atomic number 11) ? Justify your answer giving example.
Answer : (a) Electronic configuration of X(19) is 2, 8, 8, 1 and its.
(i) Valency is 1.
(ii) X is a metal.
(b) X₂O.
(c) X is more reactive than Na. X and Na belong to the same group. But Na is in the third period and X is in the fourth period. Since reactivity increases down the group X is more reactive than Na.

Question: An element ‘X’ belongs to third period and second group of the Modern Periodic Table.
(a) Write its electronic configuration.
(b) Is it a metal or non-metal ? Why ?
(c) Write the formula of the compound formed when ‘X’ reacts with an element (i) Y of electronic configuration 2, 6 and (ii) Z with electronic configuration 2, 8, 7.
Answer : (a) Electronic configuration of X is 2, 8, 2.
(b) X is metal, as it can easily loose electrons (from outer most orbit)
(c) (i) X Y                    (ii) X Z
         2 2                         2 1
Compound — XY       Compound — XZ₂

Question: Explain, giving reason, why carbon neither forms C⁴⁺ cations nor C^4– anions, but forms covalent compounds which are bad conductors of electricity and have low melting point and low boiling point.
Answer : Carbon cannot lose 4 e- to form C⁺⁴ cations, as very high energy is required to remove 4 e^– and also carbon cannot gain 4e^– to form C–4 anions as nucleus with 6 protons cannot hold 10 electrons. Therefore, it can share 4e^– to form covalent compounds.
Carbon compounds do not conduct electricity being non polar and do not form ions or charged particles. 
Due to weak intermolecular forces of attraction, carbon compounds have low melting points and boiling points.

Question: The atomic number of an element is 12.
(a) Write its electronic configuration and determine its valency.
(b) Is it more reactive or less reactive than Ca (atomic number 20) ? Justify your answer giving reason.
(c) Is it a metal or a non-metal ?
(d) Write the formula of its oxide.
Answer : (a) Electronic configuration of X(12) is 2, 8, 2 and its valency is 2.
(b) Less reactive than Ca as reactivity increases down the group.
(c) It is a metal.
(d) Formula of its oxide is XO.

Question: The atomic number of an element ‘X’ is 19.
(a) Write its electronic configuration.
(b) To which period of the Modern Periodic Table does it belong and what is its valency ?
(c) If ‘X’ burns in oxygen to form its oxide, what will be its nature – acidic, basic or neutral ?
(d) Write balanced chemical equation for the reaction when this oxide is dissolved in water. 
Answer : (a) Electronic Configuration of X (19) is 2,8,8,1.
(b) Fourth period, and its valency is 1.
(c) Basic oxide (X₂O) will be formed.
(d) X₂O + H₂O → 2XOH

Question: State the main aim of classifying elements. Which is the more fundamental property of elements that is used in the development of Modern Periodic Table ?
Name and state the law based on this fundamental property. On which side of the periodic table one can find metals, non-metals and metalloids ?
Answer : Main aim of classification of elements is the systematic study of the known elements. Atomic number is the more fundamental property of elements that is used in the development of the modern periodic table.
‘‘Properties of the elements are a periodic function of their atomic numbers.’’
Metal on the left, non-metal on the right and metalloids at the border of metals and beginning of non-metals.

Question: (a) Which two criteria did Mendeleev use to classify the elements in his periodic table?
(b) State Mendeleev’s periodic law. 
(c) Why could no fixed position be given to hydrogen in Mendeleev’s periodic table?
Answer: (a) Two criteria used by Mendeleev to classify elements are :
(i) Atomic mass
(ii) Similarity in chemical properties
(b) Mendeleev’s periodic law states that the properties of elements are the periodic function of their atomic masses.
(c)There is nofixed position given to hydrogen in Mendeleev’s periodic table because properties of hydrogen resemble both to alkali metals and to halogens.

Question. List the anomalies of Mendeleev’s periodic table which were removed in Modem Periodic Table.
Answer: Co with higher atomic mass proceeds Ni with lower atomic mass. It was solved because Co has lower atomic number than Ni.
Isotopes should have been given different slots due to different atomic mass, but it is not possible due to same chemical properties. The problem was solved because isotopes have same atomic numbers.

Question. Choose from the following:
6C, 8O, 10Ne, 11Na, 14S1
a. Elements that should be in the same period.
b. Elements that should be in the same group.
State the reason for your selection in each case.
Answer:
a. 6C(2, 4), 8O(2, 6), 10Ne(2, 8) belong to the same period i.e., 2nd period and 11Na(2, 8, 1), 14Si(2, 8, 4) belongs to the same period i.e., 3rd period.
b. 6C(2,4) and 14Si(2,8,4) belongs to the same group -14 due to same number of valence electrons, which is equal to 4.

Question. The elements X, Y and Z having atomic numbers 11, 7 and 6 respectively react with oxygen to form their oxides.
a. Arrange these oxides in increasing order of their basic nature.
b. Give reason for your answer.
Ans : X(11) is sodium and it will form Na2O, Y(7) is Nitrogen that will form N2O5, Z(6) is carbon which will form CO₂.
a. N₂O5 < CO₂ < Na₂O
b. It is because non-metallic character increases along a period, therefore basic character of oxides decreases, and acidic nature increases from left to right across the period.

Question. a. Name the element with atomic number 17.
b. To which period does it belong to?
c. To which group does it belong to?
d. Write its electronic configuration.
Answer:
a. Chlorine (17): 2, 8, 7
b. It belongs to 3rd period.
c. It belongs to group 17.
d. 2, 8, 7 is the electronic configuration of chlorine.

Question. a. Amongst the following elements identify the ones that would form anions:
K, O, Na, F, Ca, Cl, Mg
b. Write the electronic configuration of the anions identified above.
Answer:
O, F, Cl will form anions.
O2- (18): 2, 8
F-(10): 2, 8
Cl-(18): 2, 8, 8

Question. Calcium is an element with atomic number 20. Stating the reason, answer each of the following questions:
a. Is calcium a metal or a non-metal?
b. Will its atomic radius be larger or smaller than that of potassium with atomic number 19?
c. Write the formula of its oxide.
Answer:
a. Calcium is a metal because it can lose electrons to form cations.
b. Its atomic radius will be smaller due to more number of protons and electrons, more forces of attraction.
c. CaO

Question. An element M with electronic configuration (2,8,2) combines separately with NO₃ , SO₄
– 2- ^ h ^ h and PO₄
3- ^ h
radicals. Write the formula of the three compounds so formed. To which group and period of the Modern Periodic Table does the element M belong to? Will M form covalent or ionic compounds?
Answer:
M(2, 8, 2). It has valency equal to 2.

M belongs to group 2.
It belongs to 3rd period.
M will form ionic compound because M can easily lose electrons.
Bond will be formed by transfer of electrons.

Question. State the modern periodic law for the classification of elements. How many (a) groups, (b) periods are there in Modern Periodic Table?
Answer: Properties of elements are a periodic function of their atomic numbers. There are 18 groups and 7 periods in modern periodic table.

Question. How can the valency of an element be determined if its electronic configuration is known? What will be valency of an element with atomic number 9?
Answer: Valency=Number of valence electrons in case of metals and metalloids. It is also equal to 8 — Number of valence electrons in case of non-metals.
F(9) has electronic configuration of 2, 7. It is a nonmetal.
Its valency is equal to 1.

Question. An element X belongs to 3rd period and group 17 of the periodic table. State its (a) electronic configuration, (b) valency. Justify your answer with a reason.
Answer:
a. 2, 8, 7; because it has 3 shells as it belongs to 3rd period. Group 17 means 7 valence electrons.
b. Valency = 1
It can gain 1 electron to become stable i.e. to complete its octet.

Question. Choose from the following:
20Ca, 3Li, 11Na, 10Ne
a. An element having two shells completely filled with electrons.
b. Two elements belonging to same group of the periodic table.
Answer:
a. 10Ne (2, 8)
b. 3Li(2, 1) and 11Na(2, 9, 1) belong to the same group.

Question. The electronic configuration of two elements A and B are 2, 8, 7 and 2, 8, 8, 2 respectively. Write the atomic number of these elements. What will be formula of the compound formed and the nature of bond between them when these two elements chemically combine together?
Answer:

A – 2 + 8 + 7 = 17 is its atomic number.
B – 2 + 8 + 8 + 2 = 20 is its atomic number.
A(2, 8, 7) B(2, 8, 8, 2)
The nature of bond is ionic bond.

Question. The atomic number of these elements are given below:

Write the symbol of element which belongs to
a. group 1,
b. group 14 of the periodic table.
State the period of the periodic table to which these elements belong to. 
Answer:
c. A(3) has electronic configuration 2, 1. It belongs to group 1. Hence the number of valence electrons is 1.
d. B(6) has electronic configuration 2, 4. It belongs to group 14, valence electrons = 4.
They belong to second period because they have 2 shells.

Question. How the electronic configuration of the atom of an element is related to its position in the modern periodic table? Explain with one example.
Answer:
Period No. = Number of shells
Group No. = Number of valence electrons or valence electrons +10.
Example
Na(11): 2, 8,1. It has 1 valence electron, it belongs to group 1. Also Al (13): 2, 8, 3. It has 3 shells, therefore it belongs to 3rd period.

Question. The atomic number of three elements, X, Y and Z are 9, 11 and 17 respectively. Which two of these elements will show similar chemical properties? Why?
Answer:
X(9) : 2, 7
Y(11) : 2, 8, 1
Z(17) : 2, 8, 7
X and Z have similar chemical properties because they have the same number of valence electrons.

Question. In the modern periodic table, the element Calcium (atomic number = 20) is surrounded by elements with atomic numbers 12, 19, 21 and 38. Which of these elements has physical and chemical properties resembling those of Calcium and why?
Answer:
Ca(20): 2, 8, 8, 2
Mg(12): 2,8, 2
Sr(38): 2, 8, 18, 8, 2
Mg and Sr has similar properties to Ca because each of them have 2 valence electrons.

Question. Why do all elements of the
a. same group have similar properties?
b. same period have different properties?
Answer :
a. It is due to same number of valence electrons which will decide the chemical properties.
b. They differ in number of valence electrons, therefore they differ in chemical properties. They have the same number of shells.

Question. An element ‘E’ has the following electronic configuration:

a. To which group of the periodic table does element E belong to?
b. To which period of the periodic table does element E belong to?
c. State the number of valence electrons present in element E.
d. State the valency of the element E.
Answer:
a. E (2, 8, 6) belongs to group 16,
b. It belongs to 3rd period.
c. It has 6 valence electrons.
d. The valency of E is 2.

Question. An element X has mass number 35 and the number of its neutrons is 18. Identify the group number, period and valency of element X’.
Answer: X has mass number 35, number of neutrons = 18
Atomic Number = 35 – 18 = 17
a. Electronic configuration: X = 2, 8, 7 It has 7 valence electrons. It belongs to group 17.
b. It has 3 shells, therefore it belongs to 3rd period.
c. It can gain 1 electron to become stable, so its valency is equal to 1.

Question. Why is lithium with atomic number 3 and potassium with atomic number 19 are placed in group one? What will be atomic number of the first two elements in the second group?
Answer: Group 1 Group 2
Li(3): 2,1 Be(4): 2, 2
K(19):2,8,8,1 Mg (12): 2, 8, 2
Li and K are placed in group 1 due to same number of valence electrons. In second group the atomic number of first two elements will be 4 and 12 respectively.

Question. An element belongs to third period and second group of the periodic table:
a. State the number of valence electrons in it.
b. Is it a metal or a non-metal?
c. Name the element.
d. Write the formula of its oxide.
Answer:
a. It has 2 valence electrons.
b. It is a metal
c. Magnesium.
d. MgO is the formula of its oxide.

Question. An element ‘X’ has atomic number 13.
a. Write its electronic configuration.
b. State the group to which ‘X’ belongs to.
c. Is ‘X’ a metal or a non-metal?
d. Write the formula of its bromide.
Answer:
a. X(13) : 2, 8, 3
b. It belongs to group 13.
c. It is a metal.
d. XBr3 is the formula of its bromide.

Question. Give reasons:
a. Elements in a group have similar chemical properties.
b. Elements of Group 1 form ions with a charge of +1.
Answer:
a. It is due to the same number of valence electrons.
b. Group-1 elements can lose one electron to form positive ions with charge equal to +1.

Question. What is meant by periodicity of properties of elements? Why are the properties of elements placed on the same group of the periodic table similar?
Answer: The repetition of similar properties after a definite interval is called periodicity of properties. It is due to the same number of valence electrons.

Question: What physical and chemical properties of elements were used by Mendeleef in creating his periodic table? List two observations which posed a challenge to Mendeleef ’s Periodic law.
Answer: Atomic masses and similarity in physical and chemical properties are the points used by Mendeleef in creating his periodic table.
The two observations that posed a challenge in Mendeleef periodic law are :
(i) Increasing order of atomic weights could not be maintained while matching chemical properties.
(ii) Isotopes have different atomic masses but have similar chemical properties.

Question: State any three limitations of Mendeleev’s classification.
Answer: Three limitations of Mendeleev’s classification are :
(i) The position of isotopes could not be explained. Isotopes are the atoms of the same element having similar chemical properties but difierent atomic masses. If the elements are arranged according to their atomic masses, the isotopes should be placed in different groups. But isotopes were not given separate places in Mendeleev’s periodic table.
(ii) Wrong order of atomic masses of some elements could not be explained. 
When certain elements were put in their correct groups on the basis of their chemical properties, it was found in some cases that the element with higher atomic mass comes first and the element with lower atomic mass comes later.
e.g., cobalt (at. wt. = 58.9) comes before nickel (at. wt. = 58.7).
(iii) There is no fixed position given to hydrogen in Mendeleev’s periodic table because properties of hydrogen resemble both to alkali metals and to halogens.

Question: Write the names given to the vertical columns and horizontal rows in the Modern Periodic Table. How does the metallic character of elements vary on moving down a vertical column? How does the size of atomic radius vary on moving left to right in a horizontal row? Give reason in support of your answer in the above two cases. 
Answer: In Modern periodic table, there are 18 vertical columns called groups and 7 horizontal rows called periods.
The elements which have a greater tendenc  to loose electrons are more metallic thus, the metallic character of elements increases down the group as their tendency to loose electrons increases.Atomic radius decreases as we move from left to right in a horizontal row. At each successive element, the electron enters to the same shell due to which there is increase in nuclear charge and the electrons are pulled with greater attractive force. Hence, the atomic size decreases.

Question: Write the number of periods and groups in the Modern Periodic Table. How does the metallic character of elements vary on moving (i) from left to right in a period, and (ii) down a group?
Answer: In the Modern periodic table, there are 18 vertical columns called groups and 7 horizontal rows called periods. Trend of metallic character :
(i) Along the period from left to right : Metallic character of elements decreases as we move from left to right in a period. Metallic character depends on the electropositive character (tendency to loose electrons) of the elements. As we go across the period from left to right, one electron is added to same shell at every stage which increases the effective nuclear charge and hence, valence electrons becomes more and more closer to the nucleus. Due to this, the tendency of atoms to loose valence electrons and form positive ions decreases.
Hence, electropositive character decreases resulting in decrease of metallic character.
(ii) Down the group : Metallic character of elements increases on moving down the group as the electropositive character increases down the group.

Question: An element P (atomic number 20) reacts with an element Q (atomic number 17) to form a  compound. Answer the following questions giving reason : Write the position of P and Q in the Modern Periodic Table and the molecular formula of the compound formed when P reacts with Q .
Answer: Atomic number of P = 20
∴ Electronic configuration of P = 2, 8, 8, 2
Atomic number of Q = 17
∴ Electronic configuration of Q = 2, 8, 7
As P contains 4 shells, it belongs to 4th period and due to presence of two valence electrons, it belongs to 2nd group.
Similarly, Q contains 3 shells and 7 valence electrons thus, it belongs to 3rd period and 17th (10 + 7) group.
The molecular formula of compound formed when P reacts with Q will be :

Question: Calcium is an element with atomic number  20. Stating reason answer each of the following questions :
(i) Is calcium a metal or non-metal?
(ii) Will its atomic radius be larger or smaller than that of potassium with atomic number 19?
(iii) Write the formula of its oxide.
Answer: Given that, atomic number of calcium is 20. So, its electronic configuration = 2, 8, 8, 2
(i) As, it has 2 valence electrons in the outermost shell which can be easily lost, so it is a metal.
(ii) Atomic number of K (potassium) is 19 so, it is placed before Ca(20) in the same period. On moving from left to right in a period, the atomic radius decreases. Hence, atomic radius of Ca(20) will be smaller than that of K(19).
(iii) The valency of calcium as well as oxygen is 2 thus, the formula of the oxide will be CaO.

Question: Name any two elements of group one and write their electronic configurations. What similarity do you observe in their electronic
configurations? Write the formula of oxide of any of the aforesaid element.
Answer: Two elements of group 1 are sodium (Na) and potassium (K).
Electronic configuration of Na (11) = 2, 8, 1
Electronic configuration of K (19) = 2, 8, 8, 1
From the electronic configuration, we observe that
both (Na and K) have one electron in outermost
shell due to which they have valency equal to one.
Thus, formula of their oxides are, Na₂O and K₂O.

Question:The position of eight elements in the Modern Periodic Table is given below where atomic numbers of elements are given in the parenthesis. table

(i) Write the electronic configuration of Ca.
(ii) Predict the number of valence electrons in Rb.
(iii) What is the number of shells in Sr?
(iv) Predict whether K is a metal or a nonmetal?
(v) Which one of these elements has the largest atom in size?
(vi) Arrange Be, Ca, Mg and Rb in the increasing order of the size of their respective atoms. 
Answer: (i) Atomic number of Ca = 20 ∴ Electronic conffguration = 2, 8, 8, 2
(ii) Rb (37), electronic conffguration = 2, 8, 18, 8, 1 Thus, number of valence electrons = 1
(iii) As Sr (38) belongs to period number 5 so, it will have 5 shells.
(iv) As K(19) = 2, 8, 8, 1 So, it has 1 valence electron which can be easily lost to attain the noble gas conffguration. Hence, potassium (K) is a metal.
(v) Size of the atom increases down the group and decreases from left to right along a period. Thus, Rb (37) will be the largest atom among given elements.
(vi) Increasing order of atomic size is Be < Mg < Ca < Rb

Question: Three elements ‘X’, ‘Y’ and ‘Z’ have atomic numbers 7, 8 and 9 respectively.
(a) State their positions (group number and period number both) in the Modern Periodic Table.
(b) Arrange these elements in the decreasing order of their atomic radii.
(c) Write the formula of the compound formed when ‘X’ combines with ‘Z’.
Answer: (a) For element X of atomic number 7, the electronic configuration is 2, 5 so it has 5 valence electrons and hence, it belongs to group 15. As seven electrons are filled in two shells so, it belongs to 2^nd period.
Similarly, for Y(8), electronic configuration = 2, 6 Period number = 2, Group number = 16 and for Z(9) = 2, 7
Period number = 2, Group number = 17(b) As size of the atoms decreases on moving from left to right in a period so, the order of atomic radii will be : X > Y > Z
(c) Formula of the compound when X combines with Z : 

Question: State the main aim of classifying elements. Which is the more fundamental property of elements that is used in the development of Modern Periodic Table? Name and state the law based on this fundamental property. On which side of the periodic table one can find metals, non-metals and metalloids?
Answer: The main aim of classifying elements is the prediction of their properties with more precision (systematic study of known elements).
When the elements are arranged on the basis of increasing atomic number then it is easier to predict their properties. This led to the development of Modern periodic table.
– Modern periodic law states that the properties of elements are periodic function of their atomic numbers.
– In Modern periodic table, the metals like sodium and magnesium are towards left hand side while the non-metals like sulphur and chlorine are found on the right hand side.
Elements like silicon, germanium, etc. which lie along the border line (group 13 to group16) are semi-metals or metalloids because they exhibit some properties of both metals and non-metals.

Question: How many groups and periods are there in the Modern Periodic Table? How do the atomic size and metallic character of elements vary as we move :
(a) down a group and
(b) from left to right in a period?
Answer: There are 18 groups and 7 periods in the Modern periodic table.
– Atomic size increases down the group, while moving from left to right in a period it decreases.
– Metallic character of elements increases down the group while moving from left to right in a period it decreases.

Question:The electrons in the atoms of four elements A, B, C and D are distributed in the three shells having 1, 3, 5 and 7 electrons in the outermost shell respectively. State the period in which these elements can be placed in the Modern Periodic Table. Write the electronic configuration of the atoms of A and D and the molecular formula of the compound formed when A and D combine. 
Answer: P, Q, R and S all belong to 3rd period so, all of them will have 3 shells and the number of electrons in their outermost shell is 1, 3, 5 and 7 respectively.
∴ Electronic configuration of Q = 2, 8, 3 and its valency = 3
Similarly, electronic configuration of R = 2, 8, 5 and its valency = 8 – 5 = 3
Electronic configuration of P = 2, 8, 1
Thus, valency of P = 1
Electronic configuration of S = 2, 8, 7
Thus, valency of S = 8 – 7 = 1
Molecular formula of the compound :

Question: What is meant by ‘group’ in the Modern Periodic Table? How do the following change on moving from top to bottom in a group?
(i) Number of valence electrons.
(ii) Number of occupied shells.
(iii) Size of atoms.
(iv) Metallic character of elements.
(v) Efiective nuclear charge experienced by valence electrons. 
Answer: The vertical columns in the Modern periodic table are called groups. ere are total 18 groups in the Modern periodic table.
(i) In a particular group, the number of valence electrons remains the same.
(ii) On moving down the group, there is addition of an extra shell successively. Hence, number of occupied shells increases.
(iii) Due to addition of extra shells down the group, the size of the atoms i.e., the distance between nucleus and the outermost shell also increases.
(iv) Down the group as atomic size increases, the outermost electron is pulled by nucleus to lesser extent and hence, tendency to loose electrons
increases i.e., metallic character increases. 
(v) Efiective nuclear charge experienced by valence electrons decreases down the group due to increase in size of atoms. 

Question: The elements Be, Mg and Ca each having two electrons in their outermost shells are in periods 2, 3 and 4 respectively of the Modern Periodic Table. Answer the following questions, giving justification in each case :
(i) Write the group to which these elements belong.
(ii) Name the least reactive element.
(iii) Name the element having largest atomic radius.
Answer: (i) As Be, Mg and Ca have two electrons in their outermost shell so, they all belong to group 2.
(ii) Be will be least reactive element, as down the group the reactivity of the elements increases.
Be being smaller in size as compared to others will have less tendency to loose electrons and hence, is less reactive.
(iii) As we move down the group, atomic radius increases hence, calcium will have the largest atomic radius. 

Question: Write the number of groups and periods in the Modern Periodic Table. Mention the criteria of placing elements in the (i) same group and (ii) same period. Illustrate your answer with an example for each case.
Answer: There are 18 groups and 7 periods in the Modern periodic table.
(i) For elements to be in the same group, they should have same number of electrons in their outermost shells. For example, sodium and potassium have one electron in their outermost shells, so they belong to same group i.e., group 1.
(ii) For elements to be in the same period, they should have same number of shells. For example, magnesium (12) and aluminium (13) contain three shells so, they belong to period 3.

Question: What are groups and periods in the periodic table? Two elements X and Y belong to group 1 and 2 respectively and are in the same period of the periodic table. How do the following properties of X and Y vary?
(i) Size of their atoms.
(ii) Their metallic character.
(iii) Their valencies in forming oxides.
(iv) Molecular formula of their chlorides.
Answer: The horizontal rows of elements in the periodic table are called periods.there are seven periods in the long form of periodic table.
The vertical columns in a periodic table are called groups. There are 18 groups in the long form of periodic table.
X belongs to group-1 while Y belongs to group-2 of the same period hence, valency of X will be 1 and valency of Y will be 2.
(i) As we move along the period from left to right the size of the atoms decreases. Hence, X will be bigger than Y.
(ii) Across the period from left to right, the metallic character decreases. Hence, X is more metallic than Y.
(iii) The valency of X in its oxide will be 1 and that of Y in its oxide will be 2.
(iv) Molecular formula of their chlorides will be 

Question: Study the following table in which positions of six elements A, B, C, D, E and F are shown as they are in the Modern Periodic Table :On the basis of the above table, answer the following questions :
(i) Name the element which will form only covalent compounds.
(ii) Which element is a metal with valency one?
(iii) Which element is a non-metal with valency two?
(iv) Out of D and E, which has a bigger atomic radius and why?
(v) Write the formula of the compound formed when B combines with D.
Answer: (i) Element E will form only covalentcompounds.
(ii) Element D is a metal with valency one as it belongs to group 1.
(iii) Element B is a non-metal with valency 2 as it belongs to group 16 (valency = 8 – 6 = 2).
(iv) Out of D and E, D will have bigger atomic radius because as we move along the period from left to right there is decrease in atomic radius.
(v) Valency of B = 2 Valency of D = 1 

Question: Given below are some elements of the Modern Periodic Table :
₄Be, ₉F, ₁₄Si, ₁₉K, ₂₀Ca
(i) Select the element that has one electron in the outermost shell and write its electronic configuration.
(ii) Select two elements that belong to the same group. Give reasons for your answer.
(iii) Select two elements that belong to the same period. Which one of the two has bigger atomic size?
Answer: The electronic configurations of the given elements are :
4Be = 2, 2
9F = 2, 7
19K = 2, 8, 8, 1
20Ca = 2, 8, 8, 2
(i) Potassium (K) has one electron in its outermost shell.
(ii) Be and Ca have two electrons in their outermost shells hence, they belong to same group.
(iii) Be and F belong to the same period (period 2). K and Ca belong to the same period (period 4).
Among Be and F, Be will be bigger in size and among K and Ca, K will be bigger in size.

Question: The electronic configuration of an element is 2, 8, 8, 1.
(i) State its group number and period number in the Modern Periodic Table.
(ii) State whether this element is a metal or a non-metal. Give reason for the justification of your answer in each case. 
Answer: Electronic configuration of element = 2, 8, 8, 1
(i) It contains one electron in its outermost shell thus, it belongs to group 1. Moreover, the element has 4 shells, so it belongs to period 4.
(ii) As the element contains one electron in its outermost shell which can be easily lost hence, it acts as a metal.

Question: An element ‘X’ belongs to the third period and group one of the Modern Periodic Table. Find
(i) the number of its valence electrons
(ii) its valency, and (iii) whether X is a metal  or a non-metal. State reasons to justify your answer in each case. 
Answer: As element X belongs to group 1, thus it will have one electron in its outermost shell. Moreover, it belongs to period 3 which implies that X has 3 shells.
(i) Electronic configuration of X will be 2, 8, 1 Hence, number of valence electrons = 1
(ii) Valency of X will be 1.
(iii) As X contains 1 valence electron which can be easily lost hence, it is a metal.

Question: (a) How many periods are there in the Modern Periodic Table of elements?
(b) How do atomic radius, valency and metallic character vary down a group?
(c) How do the atomic size and metallic character of elements vary as we move from left to right in a period?
Answer:There are 18 groups and 7 periods in the Modern periodic table.
– Atomic size increases down the group, while moving from left to right in a period it decreases.
– Metallic character of elements increases down the group while moving from left to right in a period it decreases.
Valency remains the same in a group, as the number of valence electrons are same. Valency first increases from 1 to 4 in a period and then decreases to 0.

Question:F, Cl and Br are the elements each having seven valence electrons. Which of these (i) has the largest atomic radius, (ii) is most
reactive? Justify your answer stating reason for each. 
Answer: (i) F, Cl and Br all have seven valence electrons so, they belong to the same group. On moving down the group, the atomic size of the elements increases due to addition of extra shell at each successive element. Due to this the average distance between nucleus and outermost electrons increases. Thus, Br is largest in size among F, Cl and Br.
(ii) Fluorine is the most reactive element because the chemical reactivity of non-metals decreases on going down a group as the size of the atoms goes on increasing. Hence, the attraction of incoming electrons decreases. erefore, the tendency of atoms to gain electrons decreases due to which their reactivity decreases.

Question: The atomic number of an element is 16. Predict (i) the number of valence electrons in its atom
(ii) its valency
(iii) its group number
(iv) whether it is a metal or a non-metal
(v) the nature of oxide formed by it
(vi) the formula of its chloride. 
Answer: Atomic number of element (E) = 16
∴ Electronic configuration = 2, 8, 6
(i) Number of valence electrons in the atom = 6
(ii) Valency = 8 – 6 = 2
(iii) As there are 6 valence electrons thus, its group number is 10 + 6 = 16
(iv) This element is a non-metal.
(v) The nature of oxide formed by this element is acidic.
(vi) The formula of the chloride of non-metal ‘E’ will be 

Question: The positions of three elements A, B and C in the periodic table are indicated below :
Group 16          Group 17
  –                       (First period)
– A (Second period)
– – (Third period)
B C (Fourth period)
(a) State whether element C would be a metal or a non-metal? Why?
(b) Which is the more active element A or C? Why?
(c) Which type of ion (cation or anion) will be formed by the element C? Why?
Answer:(a) C belongs to group 17 and hence, it will have 7 valence electrons in the outermost shell and has a tendency to gain electrons thus, it is a non-metal.
(b) Among A and C, A will be more reactive as the reactivity decreases down the group. So, A has more tendency to gain electrons.
(c) C will form negatively charged ion which is known as anion because group 17 elements have seven electrons in their outermost shell so, they have strong tendency to gain an electron to attain the noble gas configuration.

Question: In the following table six elements A, B, C, D, E and F (here letters are not the usual symbols of the elements) of the Modern Periodic Table with atomic number 3 to 18 are given :
(a) Which of these (i) a noble gas,(ii) a halogen?
(b) If B combines with F, what would be the formula of the compound formed?
(c) Write the electronic configurations of C and E. 
Answer: (a) (i) Noble gas = G
(ii) Halogen = F
(b) B(11) = 2, 8, 1 
F(17) = 2, 8, 7
Valency of B = 1
Valency of F = 8 – 7 = 1
Formula of the compound formed :

(c) Electronic configuration of C(12) = 2, 8, 2
 Electronic configuration of E(8) = 2, 6

Question: Atoms of eight elements A, B, C, D, E, F, G and H have the same number of electronic shells but different number of electrons in their outermost shell. It was found that elements A and G combine to form an ionic compound. This compound is added in a small amount to almost all vegetable dishes during cooking. Oxides of elements A and B are basic in nature while those of E and F are acidic. The oxide of D is almost neutral.
Based on the above information answer the following questions :
(i) To which group or period of the periodic table do the listed elements belong?
(ii) What would be the nature of compound formed by a combination of elements B and F?(iii) Which two of these elements could definitely be metals?
(iv) Which one of the eight elements is most likely to be found in gaseous state at room temperature?
(v) If the number of electrons in the outermost shell of elements C and G be 3 and 7 respectively, write the formula of the compound formed by the combination of C and G.
Answer: Eight elements A, B, C, D, E, F, G and H have same number of electronic shells. So, they belong to the same period. The biggest hint in the question is that the compound formed when A and G combine is used in almost all vegetable dishes which is NaCl. Thus, A = Na and B = Cl
(i) These elements belongs to period number 3. Group :
1        2      13      14     15      16        17      18
A       B       C        D       E        F         G        H
Na     Mg    Al       Si       P        S         Cl       Ar
(ii) The compound formed by the combination of B and F i.e., Mg and S will be ionic in nature as the bond will be formed by complete transfer ofelectrons. 

(iii) A and B i.e., sodium and magnesium will denitely be metals.
(iv) G i.e., Cl (chlorine) is found as gaseous diatomic (Cl₂) molecule at room temperature.
(v) Number of electrons in outermost shell of C = 3 Number of electrons in outermost shell of G = 7
∴ Valency of C = 3
Valency of G = 8 – 7 = 1
Thus, the formula of the compound will be 

Question: (a) What are ‘groups’ and ‘periods’ in the ‘periodic table’?
(b) Two elements M and N belong to groups I and II respectively and are in the same period of the periodic table. How do the following properties of M and N vary?
(i) Sizes of their atoms.
(ii) Their metallic characters.
(iii) Their valencies in forming oxides.
(iv) Molecular formula of their chlorides.
Answer: The horizontal rows of elements in the periodic table are called periods. There are seven periods in the long form of periodic table.
The vertical columns in a periodic table are called groups. ere are 18 groups in the long form of periodic table.
X belongs to group-1 while Y belongs to group-2 of the same period hence, valency of X will be 1 and valency of Y will be 2.
(i) As we move along the period from left to right the size of the atoms decreases. Hence, X will be bigger than Y.
(ii) Across the period from left to right, the metallic character decreases. Hence, X is more
metallic than Y.(iii) The valency of X in its oxide will be 1 and that of Y in its oxide will be 2.
(iv) Molecular formula of their chlorides will be 

Long Answer Type Questions :

Question. The electronic configuration of three elements A. B and C is given below :
A = 2. B = 2, 6 C = 2, 8, 2
(a) Which element belongs to the second period ?
(b) Which one of them is a noble gas ?
(c) What is the valency of B ?
(d) Name the element C.
(e) Which is a metal ?
Answer : (a) B belongs to second period as its valency is two.
(b) A( 2= helium) is a noble gas.
(c) 2 (8-6).
(d) Magnesium (At. No. 12)
(e) C is a metal.

Question. The electrons in the atoms of four elements A, B, C and D are distributed in three shells having 1, 3, 5 and 7, electrons respectively in their outermost shells. Write the group numbers in which these elements are placed in the Modern Periodic Table. Write the electronic configuration of the atoms of B and D, and the molecular formula of the compound formed when B and D combine. 
Answer : B – 13^th group.
               C – 15^th group.
               D – 17^th group.
Electronic configuration
B → Atomic number = 13.
              K L M
              2 8 3
D → Atomic number = 17
              K L M
               2 8 7
The molecular formula of the compound when B and D combine is BD3.

Question. List the merits and demerits of Mendeleev periodic table.
Answer : Merits of Mendeleev periodic table :
(a) At some places the order of atomic weight was changed in order to justify the chemical and physical nature.
(b) Mendeleev left some gap for new elements which were not discovered at that time.
(c) One of the strengths of Mendeleev’s periodic table was that, when inert gases were discovered they could be placed in a new group without disturbing the existing order. 
Demerits of Mendeleev periodic table :
(a) Position of hydrogen : Hydrogen resembles alkali metals (forms H+ ion just like Na+ ions) as well as halogens ( forms H- ion similar to Cl- ion).
Therefore, it could neither be placed with alkali metals (group I) nor with halogens (group VII ).
(b) Position of isotopes : Different isotopes of same elements have different atomic masses, therefore, each one of them should be given a different position in the periodic table. On the other hand, because they are chemically similar, they had to be given same position.
(c) Anomalous pairs of elements : At certain places,
an element of higher atomic mass has been placed before an element of lower atomic mass. For example, Argon (39.91) is placed before potassium (39.1).

Question. (a) List any three observations which posed a challenge to Mendeleev’s Periodic law.
(b) How does the metallic character of elements vary on moving from
(i) left to right in a period,
(ii) from top to bottom in a group of the Modern Periodic Table ?
Give reason for your answer.
Answer : (a) Three observations which posed a challenge to Mendeleev’s periodic law are :
(i) The position of isotopes could not be explained.
(ii) Wrong order of atomic masses of some elements could not be explained.
(iii) A correct position could not be assigned to hydrogen in the periodic table.
(b) (i) On moving from left to right in a period, the metallic character of elements decreases because electropositive character decreases due to increase in nuclear charge and decrease in atomic radius which causes nucleus to hold electrons more tightly.
(ii) On going down in a group of the periodic table, the metallic character of elements increases because electropositive character of elements increases with the increase in size of the atom.

Question. (a) What was the basis of Mandelẽev’s classification of elements ?
(b) List two achievements of Mandelẽev’s periodic tables.
(c) List any two observations which posed achallenge to Mandelẽev’s periodic law.
Answer : (a) Atomic mass was the basis of Mendeleev’s classification of elements.
(b) Achievements of Mendeleev’s periodic table are :
(i) He could classify all the 63 elements known at that time.
(ii) He left gaps for the yet to be discovered elements.
(iii) He predicted the properties of such elements.
(c) Observations which posed a challenge to
Mendeleev’s periodic law are :
(i) Position of isotopes.
(ii) Irregular increase in the atomic masses in going from one element to the next, making the prediction of undiscovered elements difficult.
(iii) Position of hydrogen.

Question. (a) What is a group in the periodic table ? In which part of a group would you separately expect the elements to have : (i) the greatest metallic character, (ii) the largest atomic size ?
(b) In what respects do the properties of group 1 elements differ from those of group 17 elements ?
(c) From the stand point of atomic structure, what determines which element will be the first and which is the periodic table ?
(d) Explain why, the properties of elements are repeated after 2, 8, 18 and 32 elements in the periodic table.
(e) What are the advantages of the periodic table ?
Answer : (a) The vertical columns in a periodic table are called groups.
(i) The greatest metallic character is found in the elements in the lowest part of the group.
(ii) The largest atomic size is found in the lowest part of the group.
(b) Group 1 elements have 1 valence electron and are ionic in chemical reactions, whereas, the elements of group 17 have 7 valence electrons. They all are non-metals.
(c) The number of valence electrons is the atoms of elements decides which element in a period and which will be the last in a period.
(d) The properties of elements are repeated after 2, 8, 18 and 32 elements in the periodic table because the electronic configurations of the elements are repeated in this manner.
(e) Advantages of the periodic table :
(i) It is easier to remember the properties of an element if its position in the periodic table is known.
(ii) The type of compounds formed by an element can be predicted by knowing its position in the periodic table.

Question. Why is atomic number considered to be a more appropriate parameter than atomic mass for the classification of elements in a periodic table ? How does the metallic character of elements vary as we move (a) from left to right in a period and (b) top to bottom in a group in the modern periodic table ? Give reasons to justify your answers.
Answer : Atomic number is more important parameter than atomic mass as atomic number determines the number of valence electrons which decide the chemical properties of an atom of an element. This, it is considered more appropriate. 
(a) Metallic character decreases from left to right in a period, because the tendency to lose electrons decreases due to increased attraction between nucleus and valence electrons.
(b) Metallic character increases down the group, as the tendency to lose electrons increases, due to decreased attraction between nucleus and valence electrons because outermost electrons are farther away.
Assertion and Reasoning Based Questions
Directions : In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true, but reason is not the correct explanation of assertion.
(c) If assertion is true, but reason is false.
(d) If assertion is false but reason is true.

Question. Name the element which has
a. the electronic configuration 2, 8, 1.
b. a total of two shells, with 4 electrons in the valence shell.
c. a total of three shells, with 3 electrons in the valence shell.
d. one shell which is completely filled with electrons.
e. twice as many electrons in the second shell as in the first shell.
Answer: 
a. Sodium (2, 8, -1)
b. Carbon (2, 4)
c. Aluminium (2, 8, 3) i
d. Helium (2)
e. Carbon (2, 4)

Question. a. How are the following related?
i. Number of valence electrons of different elements in the same group.
ii. Number of shells of elements in the same period.
b. How do the following change?
i. Number of shells of elements as we go down a group.
ii. Number of valence electrons of elements on moving from left to right in a period.
iii. Atomic radius in moving from left to right along a period.
iv. Atomic size as we go down a group.
Answer: 
a. i. Same number of valence electrons.
ii. Number of shells remains the same in a period.
b. i. Number of shells goes on increasing down the group.
ii. Number of valence electrons goes on increasing in a period from left to right.
iii. Atomic radius decreases along a period from left to right.
iv. Atomic size increases down a group.

Question. Explain giving justification the trends in the following properties of elements, on moving from left to right in a period in the Modern periodic Table.
(a) Variation of valency.
(b) Change of atomic radius.
(c) Metallic to non-metallic character.
(d) Electronegative character.
(e) Nature of oxides.
Answer : (a) Valency first increases, then decreases on moving from left to right in a period.
(b) Atomic radius decreases from left to right in a period.
(c) Metallic to non-metallic character increases from left to right in a period.
(d) Electronegative character increases from left to right in period.
(e) Nature of oxides changes from basic to acidic on moving from left to right in a period.

Question. a. Why do we classify elements?
b. What were the two criteria used by Mendeleev in creating his periodic table.
c. Why did Mendeleev left some gaps in his periodic table?
d. In Mendeleev’s periodic table, why was there no mention of nobles gases like He, Ne and Ar?
e. Would you place two isotopes of Cl-35 and Cl-37 in different slots because of their different atomic mass or in the same slot because their chemical properties are same?
Answer: 
a. It makes their study easier.
b. (i) Increasing order of atomic mass, (ii) Formula of oxides and hydrides.
c. The gaps were left for the elements to be discovered.
d. Noble gases were not invented at that time.
e. They will be placed at the same slot as they have the same atomic number and same chemical properties.

Question. Atoms of eight elements A, B, C, D, E, F, G and H have the same number of electronic shells but different number of electrons in their outermost shell. It was , found that elements A and G combine to form an ionic compound which can also be extracted from sea water. Oxides of the elements A and B are basic in nature while those of E and F are acidic. The oxide of element D is almost neutral. Answer the following questions based on the information given herein:
a. To which group or period of the periodic table do the listed elements belong?
b. Which one of the eight elements is likely to be a noble gas?
c. Which of the eight elements would have the largest atomic radius?
d. Which two elements amongst these are likely to be non-metals?
e. Which one of these eight elements is likely to be a semi-metal or a metalloid?
Answer: 
a. A and B belongs to group-1 and. group-2 respectively because they form basic oxides. ‘C’ belongs to group-13, ‘D’ belongs to group-14 which forms almost neutral oxide (actually amphoteric oxide), E and F belong to group-15, 16 forming acidic oxides. ‘G’ belongs to group-17 because NaCl is used in cooking. ‘H’ belongs to group 18.
they belong to third period of the periodic table.
b. H belongs to noble gas elements.
c. A will have largest atomic radius.
d. E and F are likely to be non-metals,
e. D is likely to be a metalloid or semi metal.

Question. a. The modern periodic table has been evolved through the early attempts of Dobereiner, Newlands and Mendeleev. List one advantage and one limitation of all the three attempts.
b. Name the scientists who first of all stated that atomic number of an element is a more fundamental than its atomic mass.
c. State modern periodic law.
Answer: 
(a) Dobereiner Periodic Table
Advantage: It could predict the atomic mass of middle elements quite correctly. Limitations: He could identify only three triads of elements.
Newlands Periodic Table
Advantage: Every eight element had properties similar to the first if elements are arranged in increasing order of atomic mass.
Limitations: It was applicable only upto calcium only.
No future elements could fit into it.
Mendeleev’s Periodic Table
Advantage: He could classify all the elements discovered at that time into groups and periods.He also predicted the existence of new elements which were not discovered at time.
Limitations: No fixed position of hydrogen. Position of isotopes could not be sorted out.
(b) Moseley:
Properties of elements are a periodic function of their atomic numbers.

Question. Atoms of eight elements A, B, C, D, E, F, G and H have the same number of electronic shells but different number of electrons in their outermost shell. It was found that elements A and G combine to form an ionic compound. This compound is added in a small amount to almost all vegetable dishes during cooking.
Oxides of elements A and B are basic in nature while those of E and F are acidic. The oxide of D is almost neutral. Based on the above information answer the following questions:
a. To which group or period of the Periodic Table do the listed elements belong to?
b. What would be the nature of the compound formed by the combination of elements ‘B’ and ‘F’?
c. Which two of these elements could definitely be metals?
d. Which one of the eight elements is most likely to be found in gaseous state at room temperature?
e. If the number of electrons in the outermost shell of elements ‘C’ and ‘G’ be 3 and 7 respectively, write the formula of the compound formed by the combination of‘C’and ‘G’.
Answer: 
a. A and B belongs to group-1 and group-2 respectively because they form basic oxides. ‘C’ belongs to group-13, ‘D’ belongs to group-14 which forms almost neutral oxide (actually amphoteric oxide), E and F belong to group-15, 16 forming acidic oxides. ‘G’ belongs to group-17 because NaCl is used in cooking. ‘H’ belongs to group 18.
they belong to third period of the periodic table.
b. B and F will form ionic compound because ‘B’ is a metal and ‘F’ is a non-metal.
c. A and B are definitely metals.
d. H is most likely to be found in gaseous state at room temperature.

Question. In the following table, are given eight elements A, B, C, D, E, F, G and H (here letters are not the usual symbols of the elements) of the Modem Periodic Table with the atomic numbers of the elements in parenthesis.

a. What is the electronic configuration of‘F?
b. What is number of valence electrons in ‘F’?
c. What is number of shells in *F’?
d. Arrange E, F, G, H in decreasing order of atomic size.
e. State whether F is a metal or a non¬metal.
f. Out of three elements B, E and F, which one has biggest atomic size.
Answer: 
a. 2, 8, 2 is the electronic configuration of F.
b. No. of valence electrons = 2.
c. Three shells are present in F.
d. H > G > F > E is the decreasing order of atomic size.
e. F is a metal.
f. B has biggest atomic size.

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