Atomicity (chemistry)

Atomicity is the total number of atoms present in one molecule of an element or a substance.

Example

Consider the formation of hydrogen chloride (HCl) from hydrogen (H) and chlorine (Cl). The 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.

Monatomic, diatomic and triatomic molecules of gases

A molecule which contains only one atom is called monatomic (inert gases), a molecule which contains two atoms is called diatomic (oxygen, nitric oxide, hydrogen, etc.) and a molecule containing three atoms is called a triatomic molecule (ozone, sulfur dioxide, carbon dioxide,etc.)

t Total number of atoms present in 1mole