2018 H2 Chem P1 Q6 — Periodic table Solution

The answer is D (Rb).
For the main group elements, atomic radius increases down a group (an extra electron shell each period) and decreases across a period (nuclear charge increases while the shielding stays about the same, so the valence electrons are pulled closer).
K (Group 1) and Br (Group 17) are in Period 4; Rb (Group 1) and In (Group 13) are in Period 5.
Rb is in the higher period (5 shells) and at the start of that period, where the effective nuclear charge is lowest. So Rb has the largest atomic radius.
• A is wrong: Br is in Period 4 (only 4 shells) and at the far right of the period, so its high effective nuclear charge gives it the smallest radius of the four.
• B is wrong: In is in the same period as Rb but further across (Group 13), so its greater nuclear charge pulls its valence electrons closer and it is smaller than Rb.
• C is wrong: K is in the same group as Rb but one period higher, so it has one fewer electron shell and a smaller radius than Rb.