Question

what do you mean by atomicity

Answer 1

the atomicity means the combining capacity of atoms...
example:N2,Ar...

Answer 2

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.