A mixture is a substance made up of two or more than two substances (elements or compounds) mixed together in any proportion under any conditions. Components of mixture can be separated by simple physical methods. In a mixture, properties of its components are retained.

Atomicity is the number of atoms present in the molecule. Atomicity of molecule is of different types like monoatomic, diatomic, triatomic, and polyatomic molecules. Monatomic molecule consist of one atom, diatomic molecule consists of two atoms, triatomic molecule consists of three atoms, polyatomic molecule consists of more than three atoms. For example O 2 has two atoms and is diatomic molecule, H 2 O has three atoms and is triatomic molecule, H 2 O 2 has four atoms and is polyatomic molecule.