Atomicity : The number of atoms present in one molecule of that element is called atomicity.
Monoatomic : Elements which have one atom in their molecules e.g. Helium, Neon.
Diatomic : Elements which have two atoms in their molecule e.g. Hydrogen, nitrogen, oxygen. e.g. of Determination of atomicity of a gas :
A molecules of nitrogen contains two atoms : atomicity – Diatomic
