Hello there

Here is your Answer,

Firstly Talking about Lithium and Nitrogen in terms of there Atomic Radii-

• Atomic Radii increases from top to bottom in a group as the shells increasing. Hence , Lithium has a larger atomic radius than Nitrogen because nitrogen has higher forces of interaction within its atom.

Secondly Comparison between Potassium and Chlorine on the basis of Electronegativity -

• Electronegativity of chlorine is Higher than potassium ,Due to atomic radius decrease ,then the nucleus has greater pull over electrons which causes more electro negativity.

and Lastly Comparison between Magnesium and calcium on the basis of their  Valency.

• Atomic number of magnesium is 12 and it's electronic configuration is 2,8,2. Valency is number of electrons gain or loss to achieve noble gas configuration i.e. fully filled octet  . Since magnesium has 2 electrons in it's outermost shell , hence it's valency is 2 .
• The valency of a metal is the number of it's valence electrons . Thus the valency of calcium is 2 since it has 2 valence electron.

Hope this Information will help.

Thank you :)