Ionic vs. Atomic Radii for Cations and Anions

Steps:

• Rb (Z=37) forms Rb⁺ by losing 1 electron (36 electrons); Br (Z=35) forms Br⁻ by gaining 1 electron (36 electrons).
• Cations like Rb⁺ have higher effective nuclear charge, shrinking the radius compared to neutral Rb atom.
• Anions like Br⁻ have lower effective nuclear charge per electron, expanding the radius compared to neutral Br atom.
• Thus, Rb atomic radius > Rb⁺ ionic radius; Br atomic radius < Br⁻ ionic radius.

Why D is correct:

• Cations are smaller than parent atoms due to increased effective nuclear charge; anions are larger due to electron repulsion (standard periodic trend).

Why the others are wrong:

• A: Incorrectly states Rb atomic < ionic (cations shrink) and Br atomic > ionic (anions expand).
• B: Falsely claims both atomic < ionic, ignoring cation shrinkage.
• C: Wrongly applies atomic > ionic to Br⁻, where anion expansion occurs.

Final answer: D