Please refer to Structure of Atom Class 9 Science Notes and important questions below. The Class 9 Science Chapter wise notes have been prepared based on the latest syllabus issued for the current academic year by CBSE. Students should revise these notes and go through important Class 9 Science examination questions given below to obtain better marks in exams
Structure of Atom Class 9 Science Notes and Questions
The below Class 9 Structure of Atom notes have been designed by expert Science teachers. These will help you a lot to understand all the important topics given in your NCERT Class 9 Science textbook.
Refer to Chapter 4 Structure of Atom Notes below which have been designed as per the latest syllabus issued by CBSE and will be very useful for upcoming examinations to help clear your concepts and get better marks in examinations.
- More than 2400 years ago, he named the smallest piece of matter “ATOMOS,” meaning “not to be cut.”
- Atoms were small, hard particles that were all made of the same material but were different shapes and sizes.
- Atoms were infinite in number, always moving and capable of joining together
- First recorded evidence that atoms exists.
- Using his theory, Dalton rationalized the various laws of chemical combination
Dalton’s theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights.
Limitations
- The indivisibility of an atom was proved wrong , for, an atom can be further subdivided into protons, neutrons and electrons.
- The atoms of same element are similar in all respects , but isotopes of same element have different mass.
- Dalton’s theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights.
- Discovered electrons in 1897.
- Showed us that the atom can be split into even smaller parts.
His discovery was the first step towards a detailed model of the atom.
- An atom is a uniform sphere of positive charges (due to presence of protons) as well as negative charges (due to presence of electrons).
- Atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude.
An electron is a negatively charged component of an atom which exists outside the nucleus. Each electron carries one unit of negative charge and has a very small mass as compared with that of a
JJ Thomson used cathode ray tubes to demonstrate that the cathode ray responds to both magnetic and electric fields.
Since the ray was attracted to a positive electric plate placed over the cathode ray tube (beam deflected toward the positive plate) he determined that the ray must be composed of negatively charged particles.
He called these negative particles “electrons.”
Limitation: Model failed to explain how protons and electrons were arranged in atom so close to each other.
- E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays. These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle.
- Used a Cathode Ray Tube to study “canal rays” which had electrical and magnetic properties opposite of an electron
- Canal Rays: The positively charged radiation produced in the discharge tube at low pressure and high voltage are called canal rays.
Protons
The canal rays have positively charged sub-atomic, particles known as protons (p).
Experiment: Rutherford took a thin gold foil and made alpha particles, [He2+][He2+] positively charged Helium fall on it.
| S.No. | OBSERVATION | INFERENCE |
| 1 | Most of the a-particles passed through the gold foil without getting deflected. | Most of the space inside the atom is empty. |
| 2 | Very few particles were deflected. | Positive charge of the atom occupies very little space. |
| 3 | A very few alpha particles, 1 in 100000completely rebound on hitting the goldfoil. | Nucleus of an atom is very small ascompared to the total size. |
Limitation: In Rutherford’s atomic model , Nucleus & electrons are held together by electrostatic force of attraction which would lead to the fusion between them. This does not happen in the atom.
Atomic radius ~ 100 pm = 1 x 10−10−10 m
Nuclear radius ~ 5 x 10−3−3 pm=5×10−15m5×10−15m
- In 1932, James Chadwick proved that the atomic nucleus contained a neutral particle which had been proposed more than a decade earlier by Ernest Rutherford officially discovered the neutron in 1932,
- Chadwick received the Nobel Prize in 1935.
A neutron is a subatomic particle contained in the atomic nucleus. It has no net electric, unlike the proton’s positive electric charge.
4. Sub Atomic Particles:
| Name | Symbol | Location in the atom | charge | Relative Mass | Actual Mass (g) |
| Electron | E | Around the nucleus | 1– | 1/1840 | 9.11×10−289.11×10−28 |
| Proton | P | In the nucleus | 1+ | 1 | 1.67×10−241.67×10−24 |
| Neutron | n | In the nucleus | 0 | 1 | 1.67×10−241.67×10−24 |
Protons & Neutrons collectively are known as NUCLEONS.
5. Atomic Number & Mass Number:
“Atomic number of an element is defined as the number of unit positive charges on the nucleus (nuclear charge) of the atom of that element or as the number of protons present in the nucleus.”
Atomic number, Z = Number of unit positive charge on the nucleus
= Total number of unit positive charges carried by all protons present in the nucleus.
= Number of protons in the nucleus (p)
= Number of electrons revolving in the orbits (e)
Eg :- Hydrogen – Atomic number = 1 (1 proton)
Helium – Atomic number = 2 (2 protons)
Mass number[ A] : It is defined as the sum of the number of protons & neutrons present in the nucleus of an atom.
Mass Number = Mass of protons + Mass of neutrons
Eg :- Carbon – Mass number = 12 (6 protons + 6 neutrons) Mass = 12u
Aluminium – Mass number = 27 (13 protons + 14 neutrons) Mass = 27u
6. Niel Bohr Atomic Model:
7. Electronic configuration & Valency:
Bohr and Bury Scheme – Important Rules
| S.No | where n = shell number | Maximum Capacity |
| 1 | K Shell 2 x (1) | 2 electrons |
| 2 | L Shell 2 x (2) | 8 electrons |
| 3 | M shell 2 x (3) | 18 electrons |
| 4 | N shell 2 x (4) | 32 electrons |
The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has a capacity to accommodate more electrons. This is a very important rule and is also called the OCTET RULE. The presence of 8 electrons in the outermost shell makes the atom very stable.
8. Isotopes & Isobars
ISOTOPES
| ISOTOPES | ISOBARS |
| Chemically same, physically different | Chemically different, physically same |
| Number of electrons is same | Number of electrons is different |
| Cannot be separated by chemical means | Can be separated by chemical means |
Questions for Class 9 Science Chapter 4 Structure of Atom
Question. Give the postulates of Dalton’s atomic theory.
Ans :
(i) Every element is composed of extremely small particles called atoms.
(ii) Atoms of a given element are identical, both in mass and properties. Different chemical elements have different kinds of atoms; in particular, their atoms have different masses.
(iii) Atoms cannot be created, destroyed or transformed into atoms of other elements.
(iv) Compounds are formed when atoms of different lements combine with each other in small whole number ratios.
(v) The relative number and kinds of atoms in a given compound are constant.
Question. Give the mass numbers of A and B, What is the relation between the two species?
Ans : Mass number of A = No. of protons + No. of neutrons = 6 + 6 = 12u
Mass number of B = No. of protons + No. of neutrons = 6 + 8 = 14u
The species A and B are isotopes, as they have same atomic number but different mass number.
Question. Name the three subatomic particles of an atom.
Ans : (i) Electrons (negatively charged particles) which revolve around the nucleus.
(ii) Protons (positively charged particles) which are present in the nucleus.
(iii) Neutrons (having no charge) which are present in the nucleus.
Question. Give four characteristics of isotopes.
Ans : All isotopes of an element consist of the same number of protons inside their nuclei. Hence, they have the same atomic number.
(i) All isotopes of an element consist of different number of neutrons in their nuclei. Hence, they have different mass number.
(ii) All isotopes of an element give identical chemical reactions.
(iii) Isotopes of an element have same electronic configuration.
Question. Write the conclusions drawn by Rutherford for the following observation during his scattering experiment :
(i) Most of the alpha-particles passed straight through the gold foil.
(ii) Some alpha-particles getting deflected from their path.
(iii) Very small fraction of alpha-particles getting deflected by 180°.
Ans :
(i) Most of the space inside the atom is empty.
(ii) It indicates that the positive charge of the atom occupies a very little space.
(iii) All the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
Question. What is the difference between Rutherford’s atomic model and Thomson’s atomic model?
Ans : Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits.
There is a positively charged centre in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centred in the nuclei. Thomson proposed the model of an atom to be similar to a christmas pudding. The electrons are studded like currants in a positively charged sphere like Christmas pudding and the mass of the atom wassupposed to be uniformly distributed.
Question. Describe the essential properties of the atomic nucleus. Compare these properties with the properties of electron.
Ans : Nucleus is small positively charged centre located in a very small space. An electron is a very small negatively charged particle with well established charge to mass ratio. The charge on electron forms the smallest unit of charge on atomic particles.
Question. Which is much closer to the nucleus of an atom out of K and L shells?
Ans : K shell is much closer to the nucleus of an atom.
Question. The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these elements is 40. What is the name given to such a pair of elements?
Ans : Isobars.
Question. Name the radioisotope used for examining the circulation of blood in the body.
Ans : Na-24 is the radioisotope used for examining the circulation of blood in the body.
Question. Who discovered proton?
Ans : Goldstein discovered proton.
Question. Name one element, the nucleus of which does not have any neutron.
Ans : Hydrogen.
Question. Write the names of three elementary particles which constitute an atom.
Ans : Electron, proton and neutron.
Question. Which study led to the conclusion that atoms are not indivisible?
Ans : Study of static electricity and the condition under which electricity is conducted by different substances led to the conclusion that atoms are not indivisible.
Question. What kind of elements have a tendency to gain electrons? What are they commonly called?
Ans : The elements having 5, 6 or 7 valence electrons have the tendency to gain electrons. They are commonly called non-metals.
Question. In what way the Rutherford proposed atomic model?
Ans :Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits.
There is a positively charged centre in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centered in the nucleus.
Question. Give reasons :
(i) Mass number of an atom excludes the mass of an electron.
(ii) Nucleus of an atom is charged.
(iii) Alpha-particle scattering experiment was possible by using gold foil only and not by foil of any other metal.
Ans :
(i) Mass number of an atom excludes the mass of an electron because electrons have negligible mass in comparison to protons and neutrons.
(ii) Nucleus of an atom consists of protons and neutrons. Protons are positively charged particles. So, the nucleus of an atom is charged.
(iii) Because an extremely thin film was required for the experiment and it was only possible by using gold, as gold is a highly malleable metal.
Question. Is it possible for the atom of an element to have one electron, one proton and no neutron? If so, name the element.
Ans : Yes, it is true for hydrogen atom which is represented as 11 H.
Question. Which particles were discovered by Chadwick, Thomson and Goldstein?
Ans :
(i) Chadwick – Neutron
(ii) Thomson – Electron
(iii) Goldstein – Proton
Question. Is an atom electrically neutral?
Ans : Yes, because number of protons (+ve charge) inside the nucleus are equal to the number of electrons (–ve charge) outside the nucleus.
Question. Why do some elements possess fractional atomic mass?
Ans : Some elements possess fractional atomic mass because they occur in nature in different isotopic forms. So, accordingly their average mass is calculated.
Question. How was the neutron discovered?
Ans : Atom was considered to have electrons and protons only till 1920. But electrons have negligible mass. Then entire mass of the atom was considered to be only due to the protons present in it. In 1920, Rutherford found that atomic masses of all elements are higher than the mass of all protons and electrons in their atoms. Chadwick discovered the presence of an electrically neutral particle inside the atom in 1932.
Question. Define isotopes. Why do isotopes have same atomic number but different mass number? Explain with the help of an example.
Ans : Atoms of the same element, having the same atomic number but different mass numbers are called isotopes. Isotopes have same atomic number but different mass number because they contain different number of neutrons.
For example : In nature chlorine occurs in two isotopic forms; 17Cl35 and 17Cl37.
Here atomic number of both the atoms is same but due to the difference in the number of neutrons, their mass number is different.
Question. What type of charge is present on the nucleus of an atom?
Ans : Positive charge.
Question. Explain the essential features of experiment that led to the discovery of isotopes.
Ans : The m e values of the ionized atoms were determined by studying their deflections under the influence of electrical and magnetic fields. Ionised atoms of an element do not give a unique mass and chemical properties of these atoms are identical.
Question. State the major drawback in Rutherford’s model of an atom. Mention two features of Bohr’s model which helped compensate this drawback.
Ans : The major drawback of Rutherford’s model of an atom is that it does not explain the stability of an atom. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. So, revolving electron wouldlose energy and finally fall into the nucleus.
Two features of Bohr’s model which helped to resolve this drawback : Only certain special orbits known as discrete orbits of electrons are allowed inside the atom. While revolving in these discrete orbits, the electrons do not radiate energy.
Question. What ar canal rays? Who discovered them? What is the charge and mass of canal ray?
Ans : New radiations in a gas discharge tube which are positively charged are known as canal rays. They were discovered by E. Goldstein. Charge on canal rays is positive and its mass is one unit.
Question. Define the following terms :
(i) Electronic configuration
(ii) Valence shell
(iii) Valency
Ans :
(i) The distribution of electrons amongst different orbits of an atom is known as electronic configuration.
(ii) The outermost shell of an atom is called its valence shell.
(iii) The combining capacity of an atom is called its valency or the number of electrons lost or gained by an atom to acquire noble gas configuration.
Question. Write three main features of Rutherford’s nucleus model of an atom.
Ans : On the basis of a -particle scattering experiment, Rutherford proposed a model of atom. According to him :
There is positively charged centre in an atom called the nucleus which contains the whole mass of the atom.
The electrons revolve around the nucleus.
The size of the nucleus is very small as compared to the size of the atom.
Question. What do you think would be the observation if the a -particle scattering experiment is carried out using afoil of a metal other than gold?
Ans : If experiment is done by using a foil of some other metal, then results would not have been such as most of the particles will be deflected back. Gold can be beaten to an extremely thin sheet as it is extremely malleable metal. Rutherford could predict with the use of extremely thin gold foil that, “Most of the space inside the atom is empty and nucleus is positively charged occupying a small volume within the atom.”
Question. Helium atom has two electrons in its valence shell but its valency is not two. Why?
Ans : Helium atom has two electrons in its valence shell but its valency is not two as its outermost shell is completely filled, its combining capacity or valency is zero.
Question. Write the symbols of two isotopes of uranium.
Ans : Uranium 238 (92U238) and Uranium 235 (92U235).
Question. What kind of elements have a tendency to lose electrons? What are they commonly called?
Ans : The elements having 1, 2 or 3 valence electrons have the tendency to loose electrons. They are commonly called metals.
Question. What is the relation between physical and chemical properties of isobars?
Ans : Isobars have different atomic numbers, so different electronic configuration and have different chemical properties. They are atoms of different elements, hence they have different physical properties.
Question. Is there any relationship between atomic number, mass number, isotopes, isobars and valency of an atom? Explain.
Ans : Atomic number : It tells the number of protons (Z). Atomic Mass : Total number of proton and neutron (A) is called atomic mass.
Isotopes : When atoms of same element have same number of protons but different number of neutrons is called isotopes.
Isobars : When atoms of different element have same atomic mass but different atomic number, such atoms are called isobars.
Valency : It is the combining capacity of an atom or it is defined as number of electrons lost or gained by an atom to acquire noble gas configuration.
Question. State the properties of isotopes.
Ans : Isotopes have the following uses : Treatment of disease : Radioactive isotopes are used for the treatment of dreadful diseases like cancer.
Cobalt-60 is used to kill malignant cells in patients suffering from cancer.
(i) It is used in chemical analysis.
(ii) It is used to detect disease in plants.
(iii) Dating of plants/animals being obtained by using carbon-14 after excavation.
(iv) Thyroid disorders can be treated by I-131.
(v) Leukaemia can be treated by using P-32.
(vi) Uranium-235 can produce electricity.
Question. Name the isotope used for treatment of cancer.
Ans : Isotope of cobalt : Co-60.
Question. Give the mass and charge of an electron.
Ans : (i) The mass of electron is about 9.0 × 10–31 kg.
(ii) An electron is negatively charged particle and has a charge of 1.6 × 10–19 coulomb.
Question. Find the number of neutrons in 31X15.
Ans : 31X15 indicate that no. of proton
= 15 and mass number = 31
Mass number = No. of protons
+ No. of neutrons = 31
Number of neutrons = 31 – number of protons
= 31 – 15 = 16
Question. Is C1-35 and C1-37 have different valencies?
Ans : No, it is because these are isotopes of chlorine that have same atomic number but different mass number.
Question. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Ans : The positively charged particle in the nucleus of an atom is called ‘proton’.
Question. Where is the mass of an atom concentrated?
Ans : Mass of an atom is concentrated in nucleus.
Question. Name the scientist who discovered neutrons.
Ans : J. Chadwick.
Question. What was Chadwick’s experiment?
Ans : In Chadwick’s experiment, he bombarded beryllium atoms with high speed particles.
(i) The rays emitted from beryllium during bombardment had speeded about one-tenth the speed of light.
(ii) Later Chadwick was able to show that these rays consisted of neutral particles called neutron.
Question. Which shell can accommodate a maximum of 32 electrons?
Ans : Fourth shell can accommodate a maximum of 32 electrons.
Question. Define the term “Half life”.
Ans :
Radioactive elements are unstable and disintegrate with time emitting alpha and beta particles along with gamma rays. The rate of disintegration depends on the amount of substance. As the amount of the substance decreases, disintegration becomes slower. But it takes a long time for whole of the substance to decay. Thus, the rate of radioactive decay is measured
in terms of half time. The time taken by half of the atoms of radioactive element to disintegrate is called its half-time.
Question. What is the mass of a neutron?
Ans : The mass of a neutron is 1.675 × 10–24 g.
Question. What is the charge and mass of a -particles?
Ans : Charge is 2 and mass is 4 amu.
Question. What is the charge and mass of alpha particle?
Ans : Charge = + 2 units
Mass = 4 units
Question. Explain Rutherford’s atomic model.
Ans : Rutherford purposed a model of an atom on the basis of a-particles scattering experiment. This is known as Rutherford’s nuclear model of atom.
(i) An atom consist, a heavy positively charged core called nucleus.
(ii) Nucleus is surrounded by electrons.
(iii) Electrons and nucleus are held together by electrostatic force of attraction.
(iv) Size of nucleus is very small as compared to the size of atom.
(v) Almost the entire mass of the atom is concentrated in the nucleus.
Question. Name the particles which determine the mass of an atom.
Ans : Proton and neutron are the particles which determine the mass of an atom.
Question.Why did Rutherford select a gold foil in his a-ray scattering experiment?
Ans : Rutherford selected a gold foil in his a-ray scattering experiment because gold has high malleability and can be hammered into thin sheet.
Question. Electron attributes negative charge, protons attribute positive charge. An atom has both but why there is no charge?
Ans : The negative and positive charges of electrons and protons respectively are equal in magnitude. So, the atom as a whole is electrically neutral.
Question. Write the charge and mass of an electron.
Ans : Its mass is 2000 1 times that of proton and it is negatively charged.
Question. What are nucleons?
Ans : Protons and neutrons reside in the nucleus of an atom and are thus called nucleons.
Question. Give the number of electron, proton and neutron in 59CO27 and 108Ag47.
Ans :
(i) Number of protons in Co = 27
(ii) Number of electrons in Co = 27
(iii) Number of neutrons in Co = 59 – 27 = 32
(iv) Number of protons in Ag = 47
(v) Number of electrons in Ag = 47
(vi) Number of neutrons in Ag = 108 – 47 = 61
Question. Do isotopes of an element have similar chemical properties?
Ans : Isotopes of an element have similar chemical properties because they have the same atomic number and valence electrons.
Question. Are noble gases inert?
Ans : The outermost shell of the atoms of noble gases are completely filled. So, they do not show affinity towards chemical reactions.
Question. What are valence electrons?
Ans : The electrons present in the outermost shell of an atom are known as valence electrons.
Question. Give two uses of isotopes in the field of medicines.
Ans : (i) An isotope of cobalt is used in the treatment of cancer.
(ii) An isotope of iodine is used in the treatment of goitre.
Question. What is meant by electronic configuration of elements?
Ans : The systematic distribution of electrons in various orbits of an atom is called electronic configuration of elements.
Question. Define atomic number. How it is denoted?
Ans : Number of protons of an atom determines its atomic number. It is denoted by ‘Z’.
Question. Why helium have zero valency?
Ans : Helium have zero valency because its outermost orbit is completely filled.
Question. Give the derivative source of sodium.
Ans : The symbol of Na for sodium is derived from its Latin name ‘Natrium’.
Question. Give drawbacks of Rutherford’s model.
Ans : Drawbacks of Rutherford’s model :
(i) Unable to explain the source of energy required for movement of electrons.
(ii) Unable to explain as to why a moving charge does not lose energy and fall into the nucleus.
(iii) It could not explain about the emission of radiations of different frequencies by different atoms when heated.
(iv) It could not explain the stability of an atom when charged electrons are moving under the attractive force of positively charged nucleus.
Question. Helium atom has an atomic mass of 4u and two protons in its nucleus. How many neutrons it have?
Ans : Mass number = Number of protons + Number of neutrons
4 =2 + No. of neutrons Number of neutrons = 4 – 2 = 2
Question. How can an atom become stable by losing or gaining electrons?
Ans : For stability of an atom it must have either 2 or 8 electrons in the outermost orbit. So, by losing or gaining electrons it reaches to the electronic configuration of the nearest noble gas.
Question. Give important properties of cathode rays.
Ans : (i) Cathode rays travel in straight line.
(ii) Cathode rays can rotate a light wheel placed in their path.
(iii) They ionise gas through which they pass.
(iv) They are deflected by magnetic field.
(v) They can penetrate through thin metallic sheet.
(vi) Mass of cathode ray particle is too small.
Question. How does an atom become a cation?
Ans : When an atom acquires positive charge by losing one or more electrons, it is called cation.
Question. How are the canal rays different from electrons in terms of charge and mass?
Ans : Electrons are negatively charged particles, mass of which is approximately 2000 1 that of canal rays.
Question. Write any two observations which support the fact that atoms are divisible.
Ans : Discovery of electrons and protons are two observations which support the fact that atoms are divisible.
Question. If an atom contains one electron and one proton, will it carry any charge or not?
Ans : It will not carry any charge because the positive charge on the proton neutralizes the negative charge on the electron.
Question. How has atomic number improved the definition of an element?
Ans : (i) Atomic number of an element = number of proton
= number of electron
(ii) Atomic number gives the position of the element in periodic table. An element can now be defined as a substance comprising of atoms all of which have same atomic number.
Question. Can the addition of neutron to the nucleus of an atom determine the atomic mass or number?
Ans : It will increase the atomic mass of the atom.
Question. What is an anion?
Ans : When an atom gains one or more electrons, it becomes negatively charged and is known as anion.
Question. An element 12X24 loses two electrons to form a cation which combines with the anion of element 17Y35 formed by gaining an electron.
(i) Write the electronic configuration of element X.
(ii) Write the electronic configuration of the anion of element Y.
(iii) Write the formula for the compound formed by combination of X and Y.
Ans :
(i) X = 2, 8, 2
(ii) Y = 2, 8, 8
(iii) XY2
Question. Why isotopes of an element are chemically similar?
Ans : Isotopes of an element are chemically similar because these have same electronic configuration.
Question. What is an orbit?
Ans : Orbit is the path of electron around the nucleus.
Question. (i) An ion X2+ contains 10 electrons and 12 neutrons. What is the atomic number and mass number of the element X?
(ii) Is it possible in an atom to have 12 protons and 13 electrons?
(iii) Why helium gas is inert?
Ans : (i) Atomic number = 12, Mass number = 24.
(ii) No, it is not possible. An atom is electrically neutral. The number of positively charged particles (protons) is always equal to the number of negatively charged particles (electrons).
(iii) Helium atom has completely filled outermost shell. Thus, it is inert.
Question. What were the drawbacks of Rutherford’s model of an atom?
Ans : The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration and the charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so,
the atom should be highly unstable and hence matter would not exist in the form that we know.
Question. Why do noble gases show low reactivity?
Ans : The outermost shell of the atoms of noble gases are complete. So, they show little chemical reactivity.
Question. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Ans : Number of electrons in full K-shell = 2
Number of electrons in full L-shell = 8
Total number of electrons in an atom is 2 + 8 = 10 electrons.
Question. Why an atom is electrically neutral?
Ans : An atom is electrically neutral because it contains equal number of positively charged particles and negatively charged particles, i.e. protons and electrons.
Question. If Z = 3, what would be the valency of the element? Also, name the element.
Ans : Atomic number Z = 3
Electronic configuration = 2 (K), 1 (L)
Valence shell has 1 electron in the outermost shell, so
valency of element is 1. The element is lithium.
Question. Why is the valency of Na is 1 and not 7?
Ans : It is easy for Na atom to lose one electron instead of gaining seven electrons. So, valency of Na is one and not seven.
Question. If K and L shells of an atom are full, then what would be the total number of electrons in the atom? What is the valency of this element? Name the element.
Ans : The maximum numbers of electrons that can occupy K and L shells of an atom are 2 and 8 respectively. Therefore, if K and L shells of an atom are full then the total number of electrons in the atom would be 2 + 8 = 10 electrons. So, the valency of this element is zero. The element is neon (Ne).
Question. Elaborate the postulates put forward by E. Rutherford about the structure of atom based on the a -particle scattering experiment.
Question. State the charge and mass of a neutron.
Ans : Neutron has no charge and its mass is equal to that of a proton.
Ans :
(i) Most of the space inside the atom is empty because most of the a -particles passed through the gold foil without getting deflected.
(ii) Very few particles are deflected from their path, indicating that positive charge of the atom occupies very little space.
(iii) A very small fraction of particles was deflected by 180°, indicating that all the positive charge and mass of the gold atom were concentrated in a small volume within the atom.
Question. If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom? And (ii) what is the charge on the atom?
Ans : (i) Atomic number = Number of protons = 8
(ii) The charge of the atom is zero, as total numbern of positive charge is equal to total number of negative charge. Number of protons = Number of electrons 8 = 8
Question. What is the limitation of J.J. Thomson’s model of an atom?
Ans : The major limitation of J.J. Thomson’s model is that it does not explain how positively charged particles are shielded from negatively charged particles, without getting neutralized.
Question. Name an element which has one electron, one proton and no neutron.
Ans : Hydrogen atom (1H1) has one electron, one proton and no neutron.
Question. Describe briefly Thomson’s model of an atom.
Ans : Thomson’s model of an atom : An atom consists of a positively charged sphere and the electrons are embedded in it. The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
Question. Why noble gases show least reactivity?
Ans : Noble gases have their outermost octet completely filled up, so these gases show least reactivity..
Question. Describe in brief the Rutherford’s alpha-particle scattering experiment with the help of labelled diagram. Write any three important conclusions drawn from the experiment.
Ans : Rutherford took a very thin gold foil and born bared it with a -particles and he observed that :
(i) Most of the fast moving a-particles passed straight through the gold foil.
(ii) Some of the alpha-particles were deflected by the foil by small angle.
(iii) Out of every 12000 particles, one appeared to rebound. From the above observations, he concluded :
(i) There is a positively charged centre in an atom called the nucleus. Nearly all mass of an atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in well defined orbits.
(iii) The size of the nucleus is very small as compared with the size of the atom.
Question. Why do helium, neon and argon have a zero valency?
Ans : Helium, neon and argon have 2, 8 and 8 electron in outermost shell, thus there is no need to gain or loose electrons. So, they have zero valency.
Question. An uncharged particle is found in the nucleus of an atom. Identify this uncharged particle.
Ans : Neutron.
Question. Name the scientist who concluded that the size of nucleus is very small as compared to the size of atom.
Ans : Rutherford.
Question. Helium atom has an atomic mass of 4u and two protons in its nucleus. How many neutrons does it have?
Ans : Two.
Question. From the symbol 16S32, give :
(i) Atomic number of sulphur
(ii) Mass number of sulphur
(iii) Electronic configuration of sulphur
(iv) Which of the two elements given would be chemically more reactive? S, Ar
Ans :
(i) 16
(ii) 32
(iii) Electronic configuration : 2, 8, 6.
(iv) Element S, having atomic number 16 is chemically more reactive than element Ar of atomic number 18. It is because the outermost shell of the atom of element S has six electrons only and has to complete its octet, whereas the outermost-shell of the atom of element Ar is completely filled up, i.e., its octet is complete and thus it shows little
chemical activity.
Question. Write drawbacks of Thomson’s model.
Ans : Drawbacks of Thomson’s Model : It could not explain about the stability of an atom, i.e., how both positive and negative charges could remain so close together. It could not explain the results of experiments (such as alpha ray scattering experiment) carried out by other scientists.
Question. Where is neutron located in an atom?
Ans : It is located in the nucleus of an atom.
Question. What is discharge tube?
Ans : A discharge tube is a glass tube about 70 cm long and 5 cm of diameter. Two metal electrodes are sealed at the two ends, one is connected to negative terminal of battery and other to the positive terminal. A side tube is fused at the centre of the glass tube which serves to pump out air from it, using a suction pump.
Question. Who discovered neutron?
Ans : James Chadwick.
