CBSE Class 10 Science 2019 · Paper 31/1/1/C · Question 8

(a) Lithium has the larger atomic radius. Li and N are in the same period, and across a period the rising nuclear charge ($+3$ to $+7$) pulls the same shells closer to the nucleus. (b) Chlorine is more electronegative. Chlorine ($2,8,7$) is a small atom one electron short of an octet, so it attracts electrons strongly; potassium ($2,8,8,1$) is a large atom that tends to lose its one valence electron. (c) Both have valency $2$: magnesium ($2,8,2$) and calcium ($2,8,8,2$) each have two valence electrons, being in the same group.