Question

rank the following atoms and ions in order of decreasing size.
li⁺, be²⁺, he, h⁻, b³⁺
b³⁺, be²⁺, li⁺, he, h⁻
h⁻, li⁺, he, be²⁺, b³⁺
b³⁺, be²⁺, he, li⁺, h⁻
h⁻, he, li⁺, be²⁺, b³⁺

Explanation:

Step1: Identify isoelectronic species

All species have 2 electrons (isoelectronic: same electron count).

Step2: Relate size to nuclear charge

For isoelectronic species, ionic/atomic size decreases with increasing nuclear charge (number of protons).
Proton counts:
$H^-$: 1, $Li^+$: 3, $He$: 2, $Be^{2+}$: 4, $B^{3+}$: 5

Step3: Rank by decreasing size

Larger proton count = smaller size, so order: $H^- > He > Li^+ > Be^{2+} > B^{3+}$

Answer:

H⁻, He, Li⁺, Be²⁺, B³⁺