CBSE Class 10 Science 2022 · Paper 31/6/1 · Question 2

(a) Increasing order of atomic radii: $\text{F}<\text{N}<\text{Be}<\text{Li}$, that is, F (9), N (7), Be (4), Li (3), from the smallest atom to the largest. In the 2nd period the outermost electrons of all four atoms are in the same shell, but the nuclear charge increases from $+3$ to $+9$ and pulls that shell closer, so the atomic radius decreases as the atomic number increases. (b) An element of period 3 has three shells, and one of group 13 has three valence electrons, so X has the electronic configuration 2, 8, 3 (atomic number 13, aluminium). Having three valence electrons, X has valency 3. Chlorine has valency 1, so the formula of the chloride of X is ${\text{XCl}}_3$.