Atoms and Molecules

Notes of chapter: Atoms and Molecules are presented below. Indepth notes along with worksheets and NCERT Solutions for Class 9.

(1) History-

SN Name of the scientist/Philosopher Country Time Remark by scientist /philosopher
1. Maharishi Kanand India Around 500 BC Padarth (Matter) can be divided continuously and get smallest indivisible particles named as “Permanu”.
2. Pakudha Katyayama India Around 600 BC Permanu exits in a combined form which gives us various forms of matter.
3. Democritus and Leucippus Greece Around  500 BC Called indivisible matter atom
4. Antonie L. Lavoisier France In 1789 (18th Century) Law of conservation of mass
5. Joseph L. Proust France In 1799 Laws of definite proportion
6. Dalton England 1808 Atomic theory

(2) Laws of Chemical Combination-

These laws are established by two chemist Antonie Lavoisier and Joseph L. Proust.

(i) Law of conservation of the mass states that total mass of all reactants are equal to total mass of the products in a chemical reaction.

Experiment – Take 1.25 g of copper sulphate and 1.43 g of sodium carbonate. Prepare a 5% solution of both in 10 ml water. Take small amount of the solution of sodium carbonate in a flask and weigh it. Put a cork on the mouth of the flask. Put the ignition tube in flask and weigh it also. Tilt it in the flask. When all sodium nitrate in the ignition tube mixes with copper sulphate. Weigh the flask again. The total mass of the reactants are equal to the total mass of the products.

Law of Conservation of Mass of NCERT Chapter Atoms and Molecules

(ii) Law of constant proportion states that in a chemical substance the elements are always present in definite proportions by mass.

Eg:- The ratio of mass of the hydrogen and mass of the oxygen in water is always 1:8 irrespective of source of water.

If 9 g of water decomposed then 1 g of hydrogen and 8g of oxygen are always obtained.

(3) British chemist john Dalton provided an explanation for the law of chemical combinations by his atomic theory. According to Dalton’s atomic theory, all elements of compounds are made up of small particles, ie, atoms.

Postulates of Dalton’s atomic theory

(i) All matter is made of very tiny particles called atoms, which participate in chemical reactions.

(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

(iii) Atoms of a given element are identical in mass and chemical properties.

(iv) Atoms of different elements have different masses and chemical properties.

(v) Atoms combine in the ratio of small whole numbers to form compounds.

(vi) The relative number and kinds of atoms are constant in a given compound.

(4) Atoms are smallest constituent of any matter. Radius of the atom is measured in nanometers.

1/109m = 1nm

1m = 109 nm

(5) Symbol of the elements

(i)Dalton is the first scientist to use symbols for elements which means one atom of that element. Dalton proposed symbol for 36 elements.

Some symbols for elements are given below –

Symbols for some elements as proposed by Dalton of NCERT Chapter Atoms and Molecules

(ii) Berzilius also suggested symbols for elements based on the name of the element in Latin language.

(iii) Now, IUPAC (International Union of Pure and Applied Chemistry), an international scientific organisation which approves names of elements, symbols and units.

(a) Many symbols of elements are first one or two letters of the their name in English. First letter will always written in capital letter (Uppercase) and second letter in small letter (lowercase)

Eg:- Hydrogen – H

Aluminium – Al

(b) Symbols of some elements are formed from the first letter of the name and a letter appearing later in name.

Eg:- Chlorine – Cl

Zinc – Zn

(c) Some symbols of elements are taken from their name in Latin language.

Eg:- Ferrum(Iron) Fe

Symbols and atomic masses of some elements

(6) Atomic Mass of an element is the average mass of the atoms of element.

Atomic mass can be measured in “amu”. But now, it is written as ”u” after IUPAC recommendations.

Scientist initially took 1/16 of the mass of an atom of naturally occurring oxygen as the unit because of two reasons

(a) Oxygen reacted with large number of elements and formed compounds.

(b) This atomic mass unit gave masses of most of the elements as whole numbers.

But, in 1961 carbon – 12 isotope was chosen as the standard reference for measuring atomic masses. Therefore, one atomic mass unit is a mass unit equal to exactly one – twelfth (1/12)th the mass of one atom of carbon – 12.

(7) The relative atomic mass of the atom of an element is defined as the average mass of the atom, as compared to 1/12ththe mass of one carbon – 12 atom.

(8) Atoms can not exist alone. They form molecules and ions. These molecules and ions combines and form matter that we can feel.

The smallest divisible particle of element or compound which is constituted by two or more atoms and has all properties of the pure substance is called molecule.

(9) The number of atoms constituting a molecule is known as its atomicity.


2H2  + O2 → 2H2O

Atomicity of molecules before reaction

Types of atomicity

(i) When only one atom constitutes a molecule of element is called monoatomic.

Eg:- Argon, helium etc.

(ii) When two or more atoms constitute a molecule of element is called polyatomic.

Eg:- Oxygen, chlorine etc.

(a) When only two atoms constitute a molecule of element is called diatomic.

Eg:- Oxygen etc.

(b) When only three atoms constitute a molecule of element is called triatomic.

Eg:- O3

(c) When only four atoms constitute a molecule of element is called tetra-atomic.

Eg:- Phosphorus etc.

Atomicity of some elements are tabulated below –

Type of element Name Atomicity
Non – Metal Argon Monoatomic
Helium Monoatomic
Oxygen Diatomic
Hydrogen Diatomic
Nitrogen Diatomic
Chlorine Diatomic
Ozone Triatomic
Phosphorus Tetra-atomic
Sulphur Polyatomic

(10) Molecules of compound –

When atoms of elements join together in definite proportions to form molecules, is called molecules of compound.

Eg:- Water, ammonia etc.

(11) An ion is a single charged atom or group of atoms (Molecules) that have a net charge on them.

Types of ions

(i) A negative charged ion is called anion.

Eg:- A chlorine atom gains one ion to become anion (Cl)

(ii) A positive charged ion is called cation.

Eg:- A sodium atom losses one ion to become cation (Na+)

(iii) When two or more ions considered as single unit, are called polyatomic ion.

Eg:- Hydroxide ion consists one ion of oxygen and one ion of hydrogen.

(12) A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element.

The chemical formula of a molecular compound is determined by the valency of each element.

Rules to follow while writing a chemical formula –

(i) The valences or charges on the ion mist balance.

(ii) When a compound consists of a metal and a non – metal, the name of the metal should be written first.

Eg:- Calcium Oxide (CaO)

(iii) In compounds formed with polyatomic ions, the number of ions present in the compound is indicated by enclosing the formula of ion in a bracket and writing the number of ions outside the bracket.

Eg:- Mg(OH)2

(13)Steps to write Formula of simple compounds

Eg:- Write formula of hydrogen chloride

Step 1 – Write symbols of constituent elements.

Symbol      H      Cl

Step 2 – Write valency below the symbols

Symbol      H      Cl

Valency      1       1

Step 3- Crossover the valences of the constituent atoms.. and we will get formula.

Atoms and molecules, formula of HCL, NCERT, class9

Formula of the compound will be HCl.

(14) The sum of the atomic masses of the all atoms in a molecule of the substance is called molecular mass or relative mass of a molecule of the substance.

Therefore unit for the molecular mass is atomic mass units(u).

Eg:- Calculate the relative mass (Molecular mass) of the H2.

Ans- Atomic mass of the hydrogen = 1 u

(15) The sum of the atomic masses of the all atoms in a formula unit of the ionic compound is called formula unit mass of the substance.

Eg:- Calculate the formula unit of mass of Cacl2.


Atomic mass of the Ca = 40 u

Atomic mass of the Cl = 35.5 u

Formula unit mass = Atomic mass of the Ca + 2(Atomic mass of the Cl)

(16) The word “mole was introduced by Wilhelm Ostwald in 1896 from Latin word mole meaning a heap.

One Mole of any atoms, molecules, ions or particles is that quantity in number having a mass equal to its atomic or molecular mass in grams.

1 mole = 6.022 × 1023 number of particles or ions in number

Mass of 1 mole = relative mass of particles or ions in grams

Need of mole unit – In a chemical reaction equation indicates number of atoms or molecules taking part in the reaction instead of their atomic masses.

Eg:- 2H2 + O2  →  H2O

It is clear from the equation that 2 molecule of the hydrogen reacted with one molecule of the oxygen to produce one molecule of the water.

The number of particles (atoms, molecules and ions) in 1 mole is experimentally obtained and it is fixed value, ie, 6.022 × 1023.

6.022 ×1023 is called Avogadro Constant or Avogadro number in the honour of Italian scientist Amedeo Avogadro.

1 mole = 6.022 × 1023 number of particles or ions in number

1 mole = relative mass of particles or ions in grams

Eg:- Molecular mass of the water is 18 u.

It means

18 u water has only 1 molecule of water


18 g water has 1 mole molecules of water

Relationship between Mole, Avogadro Number and Mass of NCERT Chapter Atoms and Molecules

Helping Topics

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Worksheet Class 9



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