AQA A-Level Chemistry Paper 3, 2024: Question 7

What this question tests AO1 · 1 mark

Meaning of 1st/2nd/3rd ionisation energy and what “third” specifically removes.
Using electron configuration to decide whether the 3rd electron comes from an outer shell or an inner (core) shell.
Recognising that removing a core electron causes a very large jump in ionisation energy.

Question: Which element has the highest third ionisation energy? Options: Li, Be, K, Ca. [1 mark]

B — Be

Mark scheme awards the mark for selecting B (Be).

The third ionisation energy is removing 1 electron from X²⁺(g):
X²⁺(g) → X³⁺(g) + e⁻
Ionisation energies are much higher when you start removing core (inner-shell) electrons rather than outer-shell electrons.
A huge jump happens after all valence electrons have been removed.

Write each electron configuration (at least the outer shell).
Ask: after removing 2 electrons, is the 3rd electron still in the outer shell?
If the 3rd electron would be a core electron, the third IE will be very large → likely the highest.

Choosing the alkali metal (Li or K) because “Group 1 has high ionisation energies” — actually Group 1 has lower first IEs due to easier electron removal.
Confusing “third ionisation energy” with “third electron shell”.
Assuming the largest atom (K, Ca) must have the highest IE — size trend affects outer electron removal, but this question is really about core vs outer electron removal.

No numerical calculation needed, but a clear “step sequence” secures the logic.

Write configurations:
- Li: 1s² 2s¹
- Be: 1s² 2s²
- K: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
- Ca: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
Consider after removing 2 electrons (forming X²⁺):
- Li²⁺: after removing 2 electrons from Li (2s¹ then one from 1s), the next removal is definitely from the 1s core (very high) — but note Li is very small; compare carefully with Be.
- Be²⁺: Be loses both 2s electrons to form Be²⁺ = 1s² (a helium-like core). The 3rd electron removed must be from 1s → very large 3rd IE.
- K²⁺: after removing 4s¹ and then one 3p electron, the 3rd removal is still from the n=3 outer region (not core) → not the biggest jump yet.
- Ca²⁺: after removing 4s², the 3rd removal is from 3p (still valence shell for Ca²⁺) → not core yet.
Pick the case where the 3rd electron is core: Be (Be²⁺ = 1s²) → third IE is extremely high → highest.

Don’t over-rely on periodic trends alone. The “jump” logic (core vs outer electron) is the key discriminator.
K and Ca do not hit a core electron by the 3rd IE; they are still removing electrons from the n=3 shell.

Top students explicitly link “third IE” to removing an electron from X²⁺ and identify whether that electron is inner-shell or outer-shell.
The strongest reasoning mentions that Be²⁺ has a filled 1s² shell, so the next electron removed experiences very high nuclear attraction (close to nucleus, low shielding).
Weaker answers often guess “smallest atom” without tying it to which shell is being ionised.

Because this is multiple choice, no working is required for the mark—but doing the shell check prevents slips under pressure.

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