Atoms and Molecules Class 9 Science Notes Download PDF

Please refer to Atoms and Molecules 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

Atoms and Molecules Class 9 Science Notes and Questions

The below Class 9 Atoms and Molecules 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 3 Atoms and Molecules 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.

Important Questions Class 9 Science Chapter 3 Atoms and Molecules

Limitation of “Law of definite proportion”
This law does not hold good when the compound is obtained by using different isotopes of the combining elements.

2. John Daltons Atomic Theory
Using his theory, Dalton rationalized the various laws of chemical combination which were in existence at that time. However, he assumed that the simplest compound of two elements must be binary.

3. Atoms, Molecules, Ions & Chemical Formula
Atom
An atom is the smallest particle of an element which can take part in a chemical reaction. It may or may not exist freely.

↓↓

Each atom of an element shows all the properties of the element.

Molecule
The smallest particle of matter (element or compound) which can exist in a free state.

↓↓

The properties of a substance are the properties of its molecules.

MOLECULES OF ELEMENT : The molecules of an element are constituted by the same type of atoms.
MOLECULES OF COMPOUND: Atoms of different elements join together in definite proportions to form molecules of compounds. (hetero atomic molecules)
ATOMICITY: The number of atoms contained in a molecule of a substance (element or compound) is called its atomic

Element	Formula	Atomicity
Ozone	O₃	3
Phosphorus	P₄	4
Sulphur	S₈	8
Oxygen	O₂	2

Based upon atomicity molecules can be classified as follows.
Monoatomic molecules: Noble gases helium, neon and argon exist as He Ne and Ar respectively.

Diatomic molecules: H₂,O₂,N₂,Cl₂,CO,HCl.H₂,O₂,N₂,Cl₂,CO,HCl.
Triatomic molecules: O₃,CO₂,NO₂O₃,CO₂,NO₂

SYMBOLS

The abbreviation used to represent an element is generally the first letter in capital of the English name of element.
Oxygen →→ O Nitrogen →→ N
When the names of two or more elements begin with the same initial letter, the initial letter followed by the letter appearing later in the name is used to symbolize the element
Barium →→ Ba Bismuth →→ Bi

Symbols of some elements are derived from their Latin names

ELEMENT	LATIN NAME	SYMBOL
Sodium	Natrium	Na
Copper	Cuprum	Cu
Potassium	Kalium	K
Iron	Ferrum	Fe
Mercury	Hydragyrum	Hg
Tungsten	Wolfram	W

Polyatomic Ion : A group of atoms carrying a charge is as polyatomic ion.
Eg. NH⁴⁺−Ammoniumlon:CO32−−CarbonateionNH⁴⁺−Ammoniumlon:CO32−−Carbonateion

Valency : The number of electrons which an atom can lose , gain or share to form a bond.

It is the combining capacity of an atom of the element.

Chemical Formula : A chemical formula is a short method of representing chemical elements and compounds.

Writing a Chemical Formula -CRISS-CROSS rule

RULES > All subscripts must be reduced to lowest term (except for molecular or
Examples

CATION	ANION	FORMULA	NAME
Al3+Al3+	SO42−SO42−	Al2(SO4)3Al2(SO4)3	Aluminium sulphate
Ca	HCO3−HCO3−	Ca(HCO3)2Ca(HCO3)2	Calcium bicarbonate
NH4+NH4+	Cl−Cl−	NH4ClNH4Cl	Ammonium chloride
Na+Na+	CO32−CO32−	HCO3−HCO3−	Sodium carbonate
Mg2+Mg2+	OH−OH−	Mg(OH)2Mg(OH)2	Magnesium hydroxide
Na+Na+	PO43−PO43−	Na3PO4Na3PO4	Sodium phosphate

4. Mole Concept
The mole (mol) is the amount of a substance that contains as many elementary entities as there are atoms in exactly 12.00 grams of 12C12C

The Avogadro constant is named after the early nineteenth century Italian scientist Amedeo Avogadro.

GRAM MOLECULAR MASS
Gram molecular mass is the mass in grams of one mole of a molecular substance.
Ex: The molecular mass of N₂N₂ is 28, so the gram molecular mass of N₂N₂ is 28 g.

ATOMIC MASS UNIT
An atomic mass unit or amu is one twelfth of the mass of an unbound atom of carbon-12. It is a unit of mass used to express atomic masses and molecular masses.
Also Known As: Unified Atomic Mass Unit (u).

MOLECULAR MASS : A number equal to the sum of the atomic masses of the atoms in a molecule. The molecular mass gives the mass of a molecule relative to that of the 12 C atom, which is taken to have a mass of 12.
Examples: The molecular mass of C₂H₆C₂H₆ is approximately 30 or [(2 × 12) + (6 × 1)][(2 × 12) + (6 × 1)]. Therefore the molecule is about 2.5 times as heavy as the 12C atom or about the same mass as the NO atom with a molecular mass of 30 or (14 +16).

Questions for Class 9 Science Chapter 3 Atoms and Molecules

Question. Define law of conservation of mass.
Ans : Law of conservation of mass states that, ‘Mass is neither created nor destroyed in a chemical reaction.’

Question. Name the scientists whose experimentation established laws of chemical combination. Name the laws also.
Ans : Antoine Laurent Lavoisier and Joseph L. Proust experimented and established two laws of chemical combination. These laws are : (i) Law of conservation of mass, (ii) Law of constant proportions.

Question. Give one example each of (i) Monovalent cation, (ii) Bivalent cation, (iii) Monovalent anion, (iv) Bivalent anion.
Ans : (i) K⁺ or Na⁺
(ii) Mg⁺² or Ca⁺²
(iii) F^– or C^l–
(iv) O₂– or S^2–

Question. How can Dalton’s atomic theory explain the Law of Constant Proportions?
Ans : According to Dalton’s atomic theory, atoms of the same elements are same. Also atoms combine in whole number. This means that the atoms can combine with each other in a simple fixed ratio to form molecules.

Question. What is ion?
Ans : An ion is a charged particle. It can be positive or negative.

Question. What do you mean by symbols of elements?
Ans : Each element is represented by a letter or group of two letters to write the chemical reactions conveniently. It is called symbol.

Question. How did the scientist lay the foundation of chemical sciences? Name the scientist.
Ans : Antoine Laurent Lavoisier laid the foundation of chemical science by establishing two important laws of chemical combination.

Question. An element has 8 electrons in its valence shell. What is its general name?
Ans : Noble gas.

Question. Give Latin name of Silver.
Ans : Latin name of Silver is ‘Argentum’.

Question. If 9 g of water is decomposed, how many grams of hydrogen and oxygen are obtained?
Ans : If 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.

Question. Name the two laws of chemical combination.
Ans : Law of conservation of mass and law of constant proportions.

Question. Why is it not possible to see an atom with naked eyes?
Ans : Because an atom is too small, i.e., the atomic radii of an atom is of the order 10^–19 m to 10^–9 m.

Question. Define the atomic mass unit.
Ans : The mass of th 12 1 part of C-12 is equivalent to one atomic mass unit. Previous, it was denoted by symbol ‘amu’ but nowadays it is denoted by symbol ‘u’.

Question. Which organisation approves the names of elements all over the world?
Ans : International Union of Pure and Applied Chemistry (IUPAC).

Question. What is Avogadro number?
Ans : The number 6.022 × 10²³is referred to as Avogadro number and is denoted by symbol NA.

Question. What is the similarity between chlorine molecule, nitrogen molecule and hydrogen molecule?
Ans : Chlorine molecule, nitrogen molecule and hydrogen molecule are diatomic molecules. These are formed by the union of two atoms of the same element.

Question. What is meant by a chemical formula? Give examples.
Ans : A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element, e.g., chemical formula of ammonia is NH₃, water is H₂O and carbon dioxide is CO₂.

Question. The atomic mass of an element is in fraction.” What does it mean?
Ans : If the atomic mass of an element is in fraction, this means that it exists in the form of isotopes. The atomic mass of such element is the average of atomic masses of its isotopes and is generally in fraction.

Question. When 3.0 g of magnesium is burnt in 2.00 g of oxygen, 5.00 g of magnesium oxide is produced. What mass of magnesium oxide will be formed when 3.00 g magnesium is burnt in 5.00 g of oxygen? Which law of chemical combination will govern your answer? State the law.
Ans : When 3.0 g of magnesium is burnt in 2.00 g of oxygen, 5.00 g of magnesium oxide is produced. It means magnesium and oxygen are combined in the ratio of 3 : 2 to form magnesium oxide.
Thus, when 3.00 g of magnesium is burnt in 5.00 g of oxygen, 5.00 g of magnesium oxide will be formed and the remaining oxygen will be left unused. It is governed by law of definite proportions.
It states that in a chemical substance, the elements are always present in definite proportions by mass.

Question. Calculate the following in 5.6 g of nitrogen :
(a) Number of moles of nitrogen
(b) Number of molecules of nitrogen
(c) Number of atoms of nitrogen
Ans : (a) Molar mass of nitrogen = 14 g
5.6 g of nitrogen = . 14 = 5 6 = 0.4 mole
(b) 28 g of nitrogen = 6.022 × 10²³ molecules
5.6 g of nitrogen = 2.15 × 10²² molecules
(c) 14 g of nitrogen = 6.023 × 10²³ atoms
5.6 g of nitrogen = 4.30 × 10²³ atoms

Question. Write the chemical symbols of two elements which are formed from the first letter of the elements’ name.
Ans : N (Nitrogen), F (Fluorine), I (Iodine), O (Oxygen) (any two).

Question. What is the atomicity of argon?
Ans : Mono atomic.

Question.. Distinguish between molecular mass and molar mass.
Ans : The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule, whereas the mass of 1 mole of any substance is called its molar mass

Question. Give the symbol of copper, silver, gold, oxygen, zinc.
Ans : Copper –
Cu Silver – Ag
Gold – Au
Oxygen – O
Zinc – Zn

Question. How does an atom exist?
Ans : Atom exists in the form of atom, molecule or ions.

Question. Define law of constant proportion.
Ans : Law of constant proportion states that, ‘In a pure chemical substance, the elements are always present in definite proportions by mass.’

Question. State the number of hydrogen atoms in 1 g of hydrogen.
Ans : One gram of hydrogen = One mole = 6.022 × 1023 atoms

Question. State the law of conservation of mass.
Ans : This law states that the mass can neither be created nor destroyed in a chemical reaction. That is ‘bass of reactants is always equal to mass of products.

Question. Who introduced the word ‘Mole’?
Ans : ‘Wilhelm Ostwald’ introduced the word ‘Mole’.

Question. What is Avogadro Constant?
Ans : The number of particles present in one mole of any substance is fixed with a value of 6.022 × 1023.

Question. What is the full form of IUPAC?
Ans : International Union of Pure and Applied Chemistry.

Question. Write atomicity of the following : (i) Sulphur, (ii) Phosphorus
Ans : (i) Polyatomic,
(ii) Tetra atomic.

Question. Give one relevant reason, why scientists choose th 16 1 of the mass of an atom of naturally occurring oxygen as the atomic mass unit?
Ans : Initially, 1/16th of the mass of naturally occurring oxygen was taken as the atomic mass unit because this unit gave masses of most of the elements as whole numbers.

Question. Explain the form of atoms in a solid.
Ans : A solid element is a cluster of atoms. The property of solid does not depend on a single atom but on cluster of atoms. For example : Diamond and graphite though both are composed of carbon atoms but due to different arrangements of carbon atoms in these. They are different in physical and chemical properties.

Question. Give two drawbacks of Dalton’s atomic theory.
Ans : Drawbacks of Dalton’s atomic theory :
(i) According to modern theory, atom is not the ultimate indivisible particle of matter. Atoms are divisible, i.e., they are themselves made-up of particles (protons, electrons, neutrons, etc.).
(ii) The assumption that the atoms of the same element have same mass does not hold good, in case of isotopes of an element.

Question. Calculate the mass of the following :
(i) 2 moles of carbon dioxide.
(ii) 6.022 × 1023 molecules of carbon dioxide.
Ans : (i) Molar mass of CO₂ = 12 + 2 × 16 g = 44 g
1 mole of carbon dioxide = 44 g
2 mole of carbon dioxide = 44 g × 2 = 88 g
(ii) Molar mass of CO₂ = 44 g
= 6.022×1023 molecules of carbon dioxide

Question. State two examples in each case and write their chemical formulae :
(a) Molecules having same kind of atoms only.
(b) Molecules having two different kinds of atoms.
(c) Molecules having three different kinds of atoms.
Ans : (a) F₂, Cl₂, P₄, S₈
(b) Ammonia (NH₃), Sulphur dioxide (SO₂), Carbon disulphide (CS₂).
(c) Calcium sulphate (CaSO₄), Sodium nitrate (NaNO₃).

Question. Find the number of atoms in the 0.5 mole of C atom.
Ans : 0.5 mole of C atom :
Number of atoms in 1 mole of C atom
= 6.022 × 1023 atoms
Number of atoms in 0.5 mole of C atom
= 6.022 × 1023 × 0.5
= 3.011 × 1023 atoms

Question. What are molecules? Give brief explanation of the arrangement of the constituent atoms in the molecules.
Ans : A molecule is the smallest particle of an element or compound which is stable in normal conditions. And it can freely show all the properties of that element or compound. It may be made up of one, two or more atoms. Molecule with one atom called monoatomic. E.g. helium, neon, etc. Molecule with two atoms called diatomic. E.g.
Cl₂, O₂. Similarly, there are molecules containing three atoms (CO₂), four atoms (P₄) and so on.

Question. What is the ratio by mass of combining elements in H₂O, CO₂ and NH₃ ?
Ans : H₂O ratio by mass of combining elements
= 2 : 16 = 1 : 8 (H : O)
CO₂ ratio by mass of combining elements
= 12 : 32 = 3 : 8 (C : O)
NH₃ ratio by mass of combining elements
= 14 : 3 = 14 : 3 (N : H)

Question. Write the name of the compounds : NaBr, Al2O₃, CaCO3.
Ans : NaBr = Sodium bromide
Al2O3 = Aluminium oxide
CaCO3 = Calcium carbonate

Question. What are ionic compounds?
Ans : Ionic compounds are charged particles. Such compounds form by joining or losing or sharing the electron.
For example : Sodium chloride is an ionic compound. Its constituent particles are positively charged sodium ion (Na+) and negatively charged chloride ion (Cl–).

52. What did Antoine L. Lavoisier observe regarding the formation of compound?
Ans : He noted that many compounds were composed of two or more elements. Each compound had the same elements in the same proportions.

Question. Define one mole, give its relationship with Avogadro constant.
Ans : One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams. The number of particles (atoms, molecules or ions) present in 1 mole of any substance is fixed, with a value of 6.022 × 1023. This number is called Avogadro constant or Avogadro number.

Question. Who stated the Law of Constant Proportion?
Ans : Joseph Louis Proust stated the Law of Constant Proportion.

Question. Name any two monatomic atoms.
Ans : Sodium, Aluminium.

Question. Write the value of charge of electron.
Ans : 1.6 × 10^–19 coulomb.

Question. Name the element which is used as the reference for atomic mass.
Ans : Carbon.

Question. What are polyatomic ions? Give two examples.
Ans : A group of atoms having a charge is known as polyatomic ion.
Examples : (NH₄)+ (SO₄)^2–

Question. What is meant by the term chemical formula?
Ans : The chemical formula of a compound is a symbolic representation of its composition and actual number of atoms in one molecule of a pure substance may be an atom or a compound

Question. How was the relative atomic mass determined?
Ans : Relative atomic masses were determined by using the laws of chemical combinations and the compound formed.

Question. What is the number of electrons in Mg atom and Mg2+ ion?
Ans : Mg = 12e–
Mg2+ = 10e–

Question. Define valency.
Ans : The combining power of an element to attain the noble gas configuration is called valency. Or, it is defined as number of electrons lost or gained by an atom to acquire noble gas configuration.

Question. Name the instrument which produces image of the surface of element that shows atoms.
Ans : Scanning tunnelling microscope.

Question. The relative atomic mass of oxygen atom is 16. Explain its meaning.
Ans : The relative atomic mass of an atom is the average masses of the atom, as compared to th 12 1 the mass of one carbon-12 atom.

Question. Why atoms form ions?
Ans : Atoms get stability by acquiring the stable electronic configuration of the nearest noble gas for which either they lose electrons or gain electrons and thus acquire noble gas configuration.

Question. Write the postulate given by the Indian philosopher Maharishi Kanad.
Ans : Indian philosopher Maharishi Kanad postulated if we divide matter we will get smaller and smaller particles. He said that a time will come when we come across smallest particles beyond which further division will not be possible.

Question. Who was the first scientist to give the concept of formation of compounds?
Ans : Antoine L. Lavoisier gave the concept of formation of compounds.

Question. What is the difference between an atom and molecule?
Ans : An atom is the smallest particle of an element which may or may not have independent existence. For example : Na, Al, Fe, etc. Molecule is the smallest particle of the element or compound which can exist independently. For example : O₂, H₂, N₂, etc. 25. Name two elements which have same atomic number. Ans : Two elements cannot have the same atomic number.

Question. What is molar mass? What are its units?
Ans : The mass of one mole of a substance is called its molar mass. Its unit is gram per mole (gmol^–1).Question. Give an example in each of the following cases :
(i) a divalent anion
(ii) a trivalent cation
(iii) a mono-valent anion.
Ans : (i) O^2– (ii) Fe³⁺ (iii) I^–

Question. What is the use of mole concept?
Ans : Applications of mole concept :
(i) We can calculate the number of basic particles from the number of moles as the number of moles of a substance is directly proportional to the number of elementary particles.
(ii) One mole of gas occupies 22.4 litres at 273K.
(iii) One mole of any gas occupies the same volume at same pressure and temperature.
(iv) One mole is equal to 6.022 × 1023 atoms. So, we can calculate the absolute masses of atoms and molecules.

Question. Give the postulates of Dalton’s atomic theory.Ans : Every element is composed of extremely small particles called atoms. 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.
Atoms cannot be created, destroyed or transformed into atoms of other elements. Compounds are formed when atoms of different elements combine with each 115.

Question. What is the building block of all matter?
Ans : Atom is the building block of all matter.

Question. What is the measuring unit of atomic radius?
Ans : Nanometre (nm) is the measuring unit of atomic radius.

Question. What is the difference between hydrogen chloride and nitrogen molecule formation?
Ans : Hydrogen chloride is molecular compound and formed by the union of different kinds of atoms while nitrogen is diatomic molecule and formed by union of two atoms of same kinds.

Question. Name two atoms which exist as independent atoms.
Ans : Noble gases such as argon (Ar) and helium (He) exist as independent atoms.

Question. What is the symbol of the element of molybdenum?
Ans : ‘Mo’ is the symbol of the element of molybdenum.

Question. What is the latest short form of atomic mass unit?
Ans : The latest short form of atomic mass unit is u, according to IUPAC.

Question. Write the chemical symbols of two elements which are formed from the first letter of the elements’ name.
Ans : N (Nitrogen), F (Fluorine), I (Iodine), O (Oxygen) (any two).

Question. Write the meaning of these formulae : (i) 2O, (ii) O₂, (iii) O₃
Ans : (i) 2O = Two atoms of oxygen
(ii) O₂ = One molecule of oxygen
(iii) O₃ = One molecule of ozone

Question. Write the names of the following compounds :
(a) NiS
(b) Mg(NO₃)₂
(c) Na₂SO₄
(d) Al(NO₃)₃
(e) K₃PO₄ (f) Ca3N₂
Ans : (a) Nickel sulphide,
(b) Magnesium nitrate,
(c) Sodium sulphate,
(d) Aluminium nitrate,
(e) Potassium phosphate,
(f) Calcium nitride.

Question Give the derivation source of symbol of sodium (Na).
Ans : The symbol of ‘Na’ for sodium is derived from its Latin name ‘Natrium’.

Question. Give the full form of amu?
Ans : The full from of amu is atomic mass unit.

Question. Write the symbols of tungsten and iron.
Ans : (i) Tungsten (W) and
(ii) Iron (Fe)

Question. All elements have charged valency. Explain.
Ans : No, all elements do not form ions thus they, do not have a charge. For example : Carbon has a valency of 4 and nitrogen has a valency of 3. Non-metals are formed without a charged valency. Example : In carbon tetrachloride, carbon has valency of 4 and chlorine has a valency of 1.

Question. Define the term gram atom. How is it related to mole and Avogadro number?
Ans : The atomic mass of an element expressed in grams is called gram atomic mass. One gram atom of any element contains 6.022 × 1023 atoms of the element. It is equal to one mole of atoms. One gram atomic mass = 6.022 × 1023 atoms = 1 mole

Question. Name the gas which gives the lightest positively charged particle.
Ans : Hydrogen.

Question. 50 g of 10% lead nitrate is mixed with 50 g of 10% sodium chloride in a closed vessel. It was found after reaction that 6.83 g of lead chloride was precipitated. Besides, the reaction mixture contained 90 g water and sodium nitrate. Calculate the amount of sodium nitrate formed.
Ans : 50 g of 10% lead nitrate = 5 g lead nitrate
+ 45 g water
50 g of 10% sodium chloride = 5 g sodium chloride
+ 45 g water
Total content before reaction = 5 + 5 + 90 = 100
Total content after reaction = 90 g
Amount of precipitate = 6.83 g
According to law of conservation,
Total mass of reaction mixture = 100 g
Amount of sodium nitrate = 100 – 90 – 6.83 = 3.17 g

Question. Explain the law of multiple proportions.
Ans : According to law of multiple proportions, when two elements combine to make one or more compounds then the ratio of weights of these element remain in fixed ratio to one another. For example : Hydrogen and oxygen combine to form water (H₂O) and hydrogen peroxide (H₂O₂) under different condition. 2 grams of hydrogen combines with 16 grams of oxygen in case of water while 2 grams of hydrogen combines with 32 grams of oxygen to form hydrogen peroxide. Now, the weights of oxygen combine with a fixed weight of hydrogen in water and hydrogen peroxide respectively are 16 and 32 which are in simple ratio of 16: 32 or 1 : 2.

Question. What is meant by a molecule? Give examples.
Ans : A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance, e.g., molecule of oxygen is O₂, ozone is O₃, phosphorus is P₄, sulphur is S₈, etc.

Question. Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Ans : 1 g of hydrogen reacts with oxygen = 8 g 3 g of hydrogen reacts with oxygen = 8 × 3 g= 24 g

Question. Define formula unit mass. Calculate formula unit mass of NaCl (atomic mass of Na = 23u, Cl = 35.5u).
Ans : The formula unit mass is same as molecular mass which is equal to the sum of masses of atoms present in a formula unit. Formula unit mass of NaCl = (23 + 35.5) = 58.5u.

Question. Give two examples of triatomic molecules.
Ans : Carbon dioxide (CO₂) and water (H₂O).

Question. Why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas? Explain.
Ans : The number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas because hydrogen molecule is diatomic, i.e., a molecule of hydrogen consists of two atoms of hydrogen, whereas helium is monatomic.

Question. Name the gas which gives the lightest positively charged particle.
Ans : Hydrogen.

Question. What do we get if 3 atoms of oxygen unite into a molecule, instead of usual 2 ?
Ans : We get (O3) ozone.

Question. Define atomicity.
Ans : The number of atoms present in one molecule of an element or a compound is known as its atomicity.

Question. Give difference between 2H and H₂.
Ans : 2H indicates 2 atoms of hydrogen and H₂ indicates one molecule of hydrogen.