The increasing order of the atomic radii of the following elements is :-
(a) C (b) O
(c) F (d) Cl
(e) Br

Solution

Chemistry ma’am · 4 steps

Sort by period first, since a new shell wins; then within period 2, more nuclear charge means smaller.

1. Across a period. C, O and F are all in period 2. Moving right, the nuclear charge rises but no new shell is added, so the electrons are pulled in closer. The radius therefore shrinks: F < O < C.
2. Down a group. F, Cl and Br are in group 17. Each step down adds a whole new shell, so the radius grows: F < Cl < Br.
3. Join the two. Even the largest period 2 atom (C) is smaller than the smallest period 3 atom here (Cl). The extra shell matters more than the change in nuclear charge.

   Element	Period	Group	Atomic radius
   F	2	17	64 pm
   O	2	16	66 pm
   C	2	14	77 pm
   Cl	3	17	99 pm
   Br	4	17	114 pm

4. So. The increasing order is F < O < C < Cl < Br, which is (c) < (b) < (a) < (d) < (e). That is option (b).

The idea to keep. For atomic size, a new shell beats everything else. Group by period first, then order within a period by nuclear charge: further right means smaller.

Answer from the NTA final answer key. Solution by PrepWiser.