Worked example
[PLACEHOLDER] Original exam-style question written by Chemify
Question
The first ionisation energy of aluminium is lower than that of magnesium. Explain why. [3]
Weak answer
1 / 3Aluminium has one more proton than magnesium, so it should be harder to remove an electron. But aluminium's electron is further away so it is easier to remove.
- Awarded B1Recognises that the outermost electron in Al is easier to remove.
- Missed B1Does not name the 3p subshell versus 3s.
- Missed B1Does not say the 3p electron is shielded by the 3s electrons.
Strong answer
3 / 3Al: 1s² 2s² 2p⁶ 3s² 3p¹; Mg: 1s² 2s² 2p⁶ 3s². The electron removed from Al is in the 3p subshell, which is higher in energy than 3s. The 3p electron is also partially shielded by the 3s² electrons, so less energy is needed to remove it despite the larger nuclear charge.
- Awarded B1Electron removed from 3p in Al, 3s in Mg.
- Awarded B13p is higher in energy than 3s.
- Awarded B1Extra shielding by 3s² outweighs the extra proton.
Marks follow the examiner's mark scheme: B = independent mark, M = method/reasoning, A = answer.