Atomicity (chemistry)

Atomicity is the total no. of atoms present in 1 molecule of a substance. Consider formation of 'Hydrogen Chloride' from Hydrogen and Clorine.Balanced Chemical equation can be written as:
H2 (g) + Cl2 (g) → 2HCl (g)
1 volume  + 1 volume  → 2 volumes
n molecules  + n molecules  → 2n molecules  ...(One volume of gas contains 'n' molecules.)
1 Molecules  +1 Molecules  → 2 Molecules  ...(Dividing by 'n') OR
½ Molecules + ½ Molecules  → 1 Molecules
This suggests that, 1 molecule of Hydrogen Chloride contains 1/2 molecule of hydrogen and 1/2 molecule of Chlorine. i.e. Atomicity of Hydrogen and Chlorine is 2 respectively. Also atomicity of HCl is 2.

Monoatomic, Diatomic and triatomic Molecules of gases:
molecule which contains only one atom is called monoatomic (Inert Gases) and the one which contains 2 atoms is called Diatomic (oxygen, nitric oxide, hydrogen,etc.) and the one containing 3 atoms is called triatomic molecule (Ozone, sulphur dioxide, carbon dioxide,etc.)