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