2020 H2 Chem P1 Q3 — Atomic structure Solution

The answer is B (\(\text{At}^{3+}\)).

Let \(p\), \(n\) and \(e\) be the numbers of protons, neutrons and electrons. The charge on the ion is \((p-e)+\).
\(n + p = 217\) (1)
\(n = e + 50\) (2)
Substituting (2) into (1): \(p + e = 167\).

Proton numbers from the Data Booklet: Po 84, At 85, Rn 86, Fr 87.
For \(\text{At}^{3+}\): \(p = 85\) and \(e = 85 - 3 = 82\), so \(p + e = 167\).
Then \(n = 217 - 85 = 132\) and \(n - e = 132 - 82 = 50\). Both conditions are met.

• A is wrong: \(\text{Po}^{2+}\) has \(p = 84\) and \(e = 82\), so \(p + e = 166\); it has \(n = 217 - 84 = 133\) neutrons, which is 51 more than its electrons, not 50.
• C is wrong: \(\text{Rn}^{4+}\) has \(p = 86\) and \(e = 82\), so \(p + e = 168\); it has \(n = 131\) neutrons, which is only 49 more than its electrons.
• D is wrong: \(\text{Fr}^{5+}\) has \(p = 87\) and \(e = 82\), so \(p + e = 169\); it has \(n = 130\) neutrons, which is only 48 more than its electrons.