The octet rule explains that atoms combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet i.e., eight electrons in their valence shells.
It explains the reason for chemical combinations of atoms by ionic or covalent bonds.
1. Incomplete octet: When the central atom of certain molecules has less than 8 electrons in its valence shell and yet it’s stable.
2. Expanded octet: When the central atom of certain molecules have more than 8 electrons and yet it’s stable.
3. For Hydrogen (H) to attain stability, it needs to share, gain or lose 1 electron.
4. Helium (He) has only 2 electrons and is stable.