Arrangement the following elements in increasing order of their atomic radii
(a) Li, Be, F, N
(b) Cl, At, Br, I

Explanation

(a) Atomic radii decrease along a period from left to right due to increasing nuclear charge. Li, Be F and N belong to same period. Thus, the atomic radii of Li, Be, F and N increase in the order : `F lt N lt Be lt Li`
(b) Atomic radii increase in a group from top to bottom due to the corresponding increase in the number of filled electronic shells. Cl, At, Br, I belong to same group, Thus, atomic radii of Cl,At,Br and I increase in the order : `Cl lt Br lt I lt At`.