Atoms and Molecules Class 9 Notes Science - Chapter 3

Chapter: 3

What Are Atoms And Molecules ?

Laws Of Chemical Combination

Antoine L. Lavoisier give two important laws of chemical combination.

Law of Conservation of Mass: It states that mass can neither be created nor destroyed in a chemical reaction. Law of Constant Proportions or The Law of Definite Proportions: This law was given by Proust as “In a chemical substance the elements are always present in definite proportions by mass”.

Atomic Theory

This theory was given by British chemist John Dalton. The postulates of this theory may be stated as follows:

  • All matter is made of very tiny particles called atoms.
  • Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
  • Atoms of a given element are identical in mass and chemical properties.
  • Atoms of different elements have different masses and chemical properties.
  • Atoms combine in the ratio of small whole numbers to form compounds.
  • The relative number and kinds of atoms are constant in a given compound.


Atoms are the smallest particles of an element which can take part in a chemical reaction.

Atomic mass unit: One atomic mass unit is a mass unit equal to exactly one-twelfth (1/12th) the mass of one atom of carbon-12. Example: Atomic mass of H is 1, C is 12, N is 14, O is 16 , Na is 23 etc.

Representation Of Atoms Of Different Elements


An element symbol is a one- or two-letter abbreviation for a chemical element name. When a symbol consists of two letters, the first letter is always capitalised, while the second letter is lowercase.

Symbols of Some Elements

Element Symbol
Aluminium Al
Argon Ar
Barium Ba
Boron B
Bromine Br
Element Symbol
Calcium Ca
Carbon C
Chlorine Cl
Cobalt Co
Copper Cu
Element Symbol
Fluorine F
Gold Au
Hydrogen H
Iodine I
Iron Fe
Element Symbol
Lead Pb
Magnesium Mg
Neon Ne
Nitrogen N
Oxygen O
Element Symbol
Potassium K
Silicon Si
Element Symbol
Silver Ag
Sodium Na
Element Symbol
Sulphur S
Uranium U
Element Symbol
Zinc Zn


It 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.


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

Element Symbol
Argon 1
Oxygen 2
Phosphorous 4
Sulphur 8
Iron 1


An ion is a charged particle.

Types of Ion

Cation Anion
Cannot be broken down to simpler substances. For example, copper, oxygen, iron, hydrogen, mercury, etc.
Example: Na+, Ca+2, etc.
A negatively charged ion is called an anion. Example: Cl, O–2, etc.
Chemical Formula Valency
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.

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 expressed in atomic mass units (u).

Formula Unit Mass

The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.