Inert Pair Effect: Reasons & Definitions

[johncena]

According to my textbook,inert pair effect has two definitions
1:-The occurrence of oxidation state which is two units less than the group oxidation is called inert pair effect.
2:-The reluctance of s-electrons to take part in bond formation is called inert pair effect.

But both the definitions are seeming very different with each other.
What is the reason behind inert pair effect?

 

[Char. Limit]

Well, for one thing, 2 could cause 1.

In many atoms, the s electrons are spin-paired. This causes the s orbital to be particularly stable and also prevents a s orbital from bonding. For an atom to bond with the s orbital, one of the atoms must be pushed to a different, higher-energy orbital. If the reaction can't provide this energy, it doesn't happen.