2025 O Level Chem P1 Q7 — Atomic Structure Solution

Answer: C. For each particle: protons = atomic number; neutrons = mass number − atomic number; electrons = protons − charge.

Option C: \({}^{20}_{10}\text{Ne}\) (a neutral atom). Protons = 10, neutrons = \(20 - 10 = 10\), electrons = 10 (no charge). Row matches: 10, 10, 10. Correct.

Option A: \({}^{16}_{8}\text{O}^{2-}\) has gained 2 electrons. Protons = 8, neutrons = \(16 - 8 = 8\), electrons = \(8 + 2 = 10\). Row gives electrons = 8 — wrong.

Option B: \({}^{19}_{9}\text{F}^{-}\) gained 1 electron. Neutrons = \(19 - 9 = 10\) (row says 9 — wrong).

Option D: \({}^{23}_{11}\text{Na}^{+}\) lost 1 electron. Protons = 11, electrons = 10 (row says 12 protons, 11 neutrons — both wrong).