Atomicity can be defined as the total number of atoms that composes a molecule. Simply put, it is the no of atoms present in a molecule. For example, each molecule of oxygen is composed of two oxygen atoms. So atomicity of oxygen is 2. In older contexts, atomicity is sometimes synonymous with valency.
- Inert gases like helium, neon etc are monoatomic
- Hydrogen, Oxygen is diatomic
- Ozone is triatomic
- Phosphorus(P4) is tetratomic