What is molecule

 When an atom 

combines with another 

atom (or atoms) and 

forms a compound 

it is called as molecule. A molecule is 

made up of two or more atoms chemically 

combined.

• Oxygen gas in the air that we breathe is 

made up of two oxygen atoms chemically 

combined.

• Ozone is a substance that is made up of 

three oxygen atoms chemically combined.

An atom of oxygen (O) and two atoms of 

hydrogen (H2) combine to form a molecule 

of water (H2O).

Formation of water molecule

Molecules also exhibit properties of 

matter and have individual existence. A 

molecule can be formed by the same or 

different kinds of atoms.

Molecules can be classified as follow :

• A molecule which contains only one 

atom is called monatomic molecule (inert 

gases)

• A molecule which contains two atoms is 

called diatomic molecule (oxygen, nitric 

oxide, hydrogen, etc.) 

• A molecule containing three atoms is 

called a triatomic molecule (ozone, sulphur 

dioxide, carbon dioxide, etc.)

• A molecule containing more than 3 

atoms are known as polyatomic molecule 

(phosphate, sulphur, etc.)