Q.The atomic radii of Cl, I and Br are 99, 133 and 114 pm, respectively.

Step 1. Locate each element. Cl, Br and I all belong to group 17, in periods 3, 4 and 5 respectively.

Step 2. Recall the down-group radius trend. Atomic radius increases down a group because each period adds a whole new outer shell, and the resulting rise in core shielding outweighs the simultaneous rise in nuclear charge — so effective nuclear charge $Z_{eff}$ on the valence electron actually falls (section 7.5.2.1).

Step 3. Order the values. Moving down group 17 from period 3 to period 5: Cl (99 pm) < Br (114 pm) < I (133 pm), matching Table 7.2 exactly.

Applying the down-group atomic-radius trend (section 7.5.2.1) to the group-17 halogens

• Assuming radius decreases down a group (confusing it with the across-period trend, which does decrease).
• Forgetting that these three elements are compared down ONE group, not across a period, so the reasoning must use shielding/added-shell logic, not effective-nuclear-charge-rise logic.