Define homo atomic and hetero atomic molecules.

The smallest particle of an element, which may or may not have an independent existence, is called an atom. In contrast, the smallest particle of a substance capable of independent existence is called a molecule. Thus, molecules are classified as homo atomic and heteroatomic.

1. Homo-atomic Molecule: Molecules formed by only one type of atoms are known as Homo-atomic molecule—for example, H₂, N₂, P₄ etc.
2. Hetero-atomic Molecule: Molecules formed by different types of atoms are known as Hetero-atomic molecules—for example, CO₂, NO₂, CH₄, HCl etc.