Define atomicity of a gas.

• Atomicity of a gas is defined as the number of atoms present in one molecule of a substance. For example monoatomic, diatomic, triatomic and polyatomic gases.
• Monoatomic gases composed of 1 atom like Helium ($\mathrm{He}$), Neon ($\mathrm{Ne}$) etc.
• Diatomic gases composed of 2 atoms like Hydrogen (${\mathrm{H}}_2$), Nitrogen (${\mathrm{N}}_2$) etc.
• Triatomic gases composed of 3 atoms like Ozone (${\mathrm{O}}_3$) etc.
• Polyatomic gases composed of more than 3 atoms like Phosphorus (${\mathrm{P}}_4$), Sulphur (${\mathrm{S}}_8$) etc.