2024 H2 Chem P1 Q1 — Atomic structure Solution

The answer is D (\({}^{24}\text{Mg}^{2+}\)).
A neon atom has 10 electrons, so cation X has 10 electrons, and X must also have equal numbers of protons and neutrons.
\({}^{24}\text{Mg}^{2+}\): 12 protons, \(24 - 12 = 12\) neutrons, \(12 - 2 = 10\) electrons. It is a cation, it has 10 electrons and protons = neutrons, so it fits.
• A is wrong: \({}^{16}\text{O}^{2-}\) has 10 electrons and 8 protons and 8 neutrons, but it is an anion, not a cation.
• B is wrong: \({}^{19}\text{F}^{-}\) is an anion, and it has 9 protons but \(19 - 9 = 10\) neutrons.
• C is wrong: \({}^{23}\text{Na}^{+}\) is a cation with 10 electrons, but it has 11 protons and \(23 - 11 = 12\) neutrons, so the numbers of protons and neutrons are not equal.