Atoms And Molecules Revision Notes

Notes Class 9

Please refer to the Atoms And Molecules Revision Notes given below. These revision notes have been designed as per the latest NCERT, CBSE and KVS books issued for the current academic year. Students will be able to understand the entire chapter in your class 9th Science book. We have provided chapter wise Notes for Class 9 Science as per the latest examination pattern.

Revision Notes Chapter 3 Atoms And Molecules

Students of Class 9 Science will be able to revise the entire chapter and also learn all important concepts based on the topic wise notes given below. Our best teachers for Grade 9 have prepared these to help you get better marks in upcoming examinations. These revision notes cover all important topics given in this chapter.


Conceptual levels of comprehension on the basis of feedback taken from the students

Atoms And Molecules Revision Notes

1.Laws of Chemical Combination

Atoms And Molecules Revision Notes

Verification of “Law of Conservation of mass”

A solution of sodium chloride and silver nitrate are taken separately in the two limbs of an ‘H’ shaped tube. The tube is sealed and weighed precisely. The two reactants are made to react by inverting the tube. The following reaction takes place.

AgNO3(aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)

The whole tube is kept undisturbed for sometime so that the reaction is complete.

When the tube is weighed again it is observed that:

Weight before the reaction = Weight after the reaction
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

Atoms And Molecules Revision Notes
  • 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 atomicity.

⇒ 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 .

Triatomic molecules: O3 ,CO2 , NO2.


  • 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

Atoms And Molecules Revision Notes
Atoms And Molecules Revision Notes

Polyatomic Ion : A group of atoms carrying a charge is as polyatomic ion.

eg: NH4 + Ammonium Ion ;     CO3 2- Carbonate ion

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

Atoms And Molecules Revision Notes
Atoms And Molecules Revision Notes
Atoms And Molecules Revision Notes


Al 3+SO42─Al2(SO4)3Aluminium sulphate
CaHCO3Ca(HCO3)2Calcium bicarbonate
NH4 +ClNH4ClAmmonium chloride
Na +CO32─Na2CO3Sodium carbonate
Mg2+OHMg(OH)2Magnesium hydroxide
Na+PO43─Na3PO4Sodium 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 12C

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

Atoms And Molecules Revision Notes
S.No.Symbol / formula of
atom / molecule
Atomic /molecular
mass ( u)
1mole ( in g)Avogadro no.Molar mass
( g mol -1)
1.O16 u16g6.022 x 1023 atoms16 g mol -1
2.N228 u28g6.022 x 1023 molecules28 g mol -1
3.HCl36.5 u36.5g6.022 x 1023 molecules36.5 g mol -1


Gram molecular mass is the mass in grams of one mole of a molecular substance.

Ex: The molecular mass of N2 is 28, so the gram molecular mass of N2 is 28 g.


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 12C atom, which is taken to have a mass of 12.

Examples: The molecular mass of C2H6 is approximately 30 or [(2 x 12) + (6 x 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) .

5.Molar Mass & Avogadro Constant

Atoms And Molecules Revision Notes
Atoms And Molecules Revision Notes
Atoms And Molecules Revision Notes