Octet rule : The tendency of atoms in molecules to have eight electrons in their valence shells (two for hydrogen atom) is known as the octet rule. Significance : (i) It explains the formation of most of the compounds. (ii) It is quite useful for understanding the structures of most of the organic compounds. Limitations : (i) In some compounds, the number of electrons surrounding the central atom is less than eight e.g., $\mathrm{LiCl}, \mathrm{BeH}_2$ and $\mathrm{BCl}_3$. (ii) In molecules with an odd number of electron like nitric oxide (NO) and nitrogen dioxide $(\mathrm{NO}_2)$, the octet rule is not satisfied for all the atoms. $:\mathrm{N}=^{...}\mathrm{O}:;\underset{\mathrm{:O:}}{\underset{\parallel}{\mathrm{N}}}-^{...}\mathrm{O}$ (iii) In a number of compounds there are more than eight valence electrons around the central atom. This is termed as the expanded octet. Some of the examples of such compounds are $\mathrm{PF}_5, \mathrm{SF}_6, \mathrm{H_2SO_4}$ and a number of coordination compounds.