Explain the term (a) screening effect (b) penetration effect and ( c ) metallic character.

Explanation

Screening effect: This inner electrons between the valence electron and nuclus shield the valence electron from nucleus is called shielding effect.
Penetration effect: Due to shape of the orbital, s electron penetrates nearer to the nucleus than `p`, `d` or `f`-orbitals and is more tightly held.
Metallic character: Lowering the ionisation energy, more will be the tendency to lose electron, higher will be the metallic character.