Question

rank the following atoms and ions in order of decreasing size.
br⁻, se²⁻, rb⁺, kr, sr²⁺
se²⁻, br⁻, rb⁺, sr²⁺, kr
sr²⁺, rb⁺, kr, br⁻, se²⁻
kr, br⁻, rb⁺, se²⁻, sr²⁺
se²⁻, br⁻, kr, rb⁺, sr²⁺

Explanation:

All species are isoelectronic (have 36 electrons). For isoelectronic species, ionic/atomic size decreases with increasing nuclear charge (number of protons). Proton counts: $\text{Se}^{2-}$ (34), $\text{Br}^-$ (35), Kr (36), $\text{Rb}^+$ (37), $\text{Sr}^{2+}$ (38). Larger nuclear charge pulls electrons closer, reducing size.

Answer:

D. Se²⁻, Br⁻, Kr, Rb⁺, Sr²⁺