What is inert pair effect? Explain with an example.

Inert pair effect:

• “The occurrence of oxidation states two-unit less than the group oxidation states are sometimes attributed to the inert pair effect.”
• The inert pair effect is the reluctance of 's' electrons for participating in bonding.

Example:

• Aluminum exhibits $+3$ oxidation state. With heavier elements, $+1$ oxidation state gets progressively stabilized on going down the group. This is a consequence of the so-called inert pair effect.