Atomicity is the total number of atoms present in one molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 of a substance.

Example

Consider the formation of hydrogen chloride

Hydrogen chloride

The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...

 (HCl) from hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 (H) and chlorine

Chlorine

Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

 (Cl). The balanced chemical equation

Chemical equation

A chemical equation is the symbolic representation of a chemical reaction where the reactant entities are given on the left hand side and the product entities on the right hand side. The coefficients next to the symbols and formulae of entities are the absolute values of the stoichiometric numbers...

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:

A 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.)