Atoms and Molecules,Mole Concept,Valency For Class 9 Chemistry

What is a Molecule?

  • When two or more atoms join or bond themselves chemically Molecule is formed.
  • A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.
  • Atoms of the same element or of different elements can join together to form molecules.

Molecules of an Elements

  • The Molecules of an element has same type of atoms .
  • For example . Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element.
  • a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule, O2 .
  • If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone.

The number of atoms constituting a molecule is known as its atomicity










  • Atoms of different elements join together in definite proportions to form molecules of






  • An ion is a charged particle and can be negatively or positively charged.
  • A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’.

  • Take, for example, sodium chloride (NaCl).
  • Its constituent particles are positively charged sodium ions (Na+ ) and negatively
    charged chloride ions (Cl– ).
  • Ions may consist of a single charged atom or a group of atoms that have a net charge on them.
  • A group of atoms carrying a charge is known as a polyatomic ion









Writing Chemical Formulae

  • The chemical formula of a compound is a symbolic representation of its composition.
  • The combining power (or capacity) of an element is known as its valency.
  • Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to for m a chemical compound.
  • The valencies or charges on the ion must balance.
  • When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first.
  • For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide
    (CuO) etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.
  • in compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.
  • In case the number of polyatomic ion is one, the bracket is not required. For example, NaOH.






For Hydogen chloride the valency of Hydrogen is 1 and valency of oxygen is 1 .






Valency of Hydrogen is 1 and Sulphur is 2. As they share ions The formula becomes H2S










The valencies of the two elements are the same. You may arrive at the formula Ca2O2. But we simplify the formula as CaO.











Magnesium Nitrate Mg(NO3)2

Molecular Mass
The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.

  • It is therefore the relative mass of a molecule expressed in atomic mass units (u).

Let us Understand Molecular Mass by taking an example

(a) Calculate the relative molecular mass of water (H 2O).
(b) Calculate the molecular mass of HNO3.

(a) Atomic mass of hydrogen = 1u, oxygen = 16 u
So the molecular mass of water, which contains two atoms of hydrogen and one atom of oxygen is = 2 × 1+ 1×16 = 18 u
(b) The molecular mass of HNO3 = the atomic mass of H + the atomic mass of N+ 3 × the atomic mass of O = 1 + 14 + 48 = 63 u

Formula Unit Mass

  • Formula unit mass is calculated in the same manner as we calculate the molecular mass
  • We use this term the word formula unit for those substances whose constituent particles are ions
  • For example, sodium chloride which has ions in it has a formula unit NaCl.
  • Its formula unit mass can be calculated as– 1 × 23 + 1 × 35.5 = 58.5 u

Now Let’s learn about Mole Concept

  • Let us Take one Example of reaction with hydrogen and water
                               2H2+ O2 → 2H2O
  • In the Above reaction two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water
  • 4 u of hydrogen molecules combine with 32 u of oxygen molecules to form 36 u of water molecules.
  • the quantity of a substance can be characterized by its mass or the number of molecules
  • a chemical reaction equation shows directly the number of atoms or molecules taking part or present in the reaction.
  • Therefore, it is better to refer to the quantity of a substance in terms of the number of its molecules or atoms, rather than their masses.
  • So, a new unit “mole” was introduced

  • One mole of any species (atoms,molecules, ions or particles) is the quantity in numbers of that mass unit
  • 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 is an experimentally obtained number and it has a fixed value of 6.022 × 1023 and is called Avogadro Number
  • Taking an example here ,the atomic mass of a single carbon atom is 12 unit. which is 1 mole and it has 6.022 × 1023.
  • Similarly 18 u water has only 1 molecule of water, 18 g water has 1 mole molecules of water, that is, 6.022 × 1023 molecules of water

Now let us understand the mole concept by doing another Problem

Calculate the number of moles for the following:
(i) 52 g of He (finding mole from mass)
(ii) 12.044 × 1023 number of He atoms (finding mole from number of particles).

No. of moles = n
Given mass = m
Molar mass = M
Given number of particles = N
Avogadro number of particles = N0
(i) Atomic mass of He = 4 u
Molar mass of He = 4g
Thus, the number of moles











2. Calculate the mass of the following:
(i) 0.5 mole of N2 gas (mass from mole of molecule)
(ii) 0.5 mole of N atoms (mass from mole of atom)
(iii) 3.011 × 1023 number of N atoms (mass from number)
(iv) 6.022 × 1023 number of Nmolecules (mass from number)

(i) mass = molar mass × number of moles
⇒m=M× n= 28 × 0.5 =14g
(ii) mass = molar mass × number of moles
⇒ m = M × n = 14 × 0.5 = 7 g












Now Answer the Following Questions

  1. What is ION
  2. What is Valency
  3. Define molecular mass of a substance
  4. Write the FullForm of IUPAC
  5. Calculate the Molecular Mass of Ethanol?C2H5OH

Click here to visit and download Free NCERT Solutions on Atoms and Molecules

Click Here to Read the First Part of the Lesson

Scroll to Top

Sign in to edusaint

Login with your Social Account