2020 H2 Chem P1 Q2 — Atomic structure Solution

The answer is D (Ar).

Successive ionisation energies rise gradually while electrons are removed from the same shell, then jump sharply when the next electron must come from an inner shell, which is closer to the nucleus and less shielded.
• 5th to 8th: 7200, 8800, 12 000, 14 000 \(\text{kJ mol}^{-1}\), a gradual rise.
• 8th to 9th: 14 000 to 41 000 \(\text{kJ mol}^{-1}\), almost three times larger.

So the 9th electron is the first one removed from the inner (n = 2) shell, and the element has 8 electrons in its outer shell (\(3s^2 3p^6\)). A Period 3 element with 8 outer electrons is in Group 18: argon.

• A is wrong: P has 5 outer electrons (\(3s^2 3p^3\)), so its big jump would be between the 5th and 6th ionisation energies; here the 5th to 6th rise is only from 7200 to 8800 \(\text{kJ mol}^{-1}\).
• B is wrong: S has 6 outer electrons, so its big jump would be between the 6th and 7th ionisation energies; here 8800 to 12 000 \(\text{kJ mol}^{-1}\) is only a gradual rise.
• C is wrong: Cl has 7 outer electrons, so its big jump would be between the 7th and 8th ionisation energies; here 12 000 to 14 000 \(\text{kJ mol}^{-1}\) is only a small rise.