AQA A-Level Chemistry AS Paper 1, June 2022: Question 12
1 mark · Medium difficulty · Multiple Choice
Identify which pair has an atom Y with a greater first ionisation energy than atom X based on their electron configurations.
Practise this questionQuestion
Question text
12 In which pair is the first ionisation energy of atom Y greater than that of atom X?
[1 mark]
Electron Electron
configuration configuration
of atom X of atom Y
A 1s22s2 1s22s22p1
B 1s22s22p3 1s22s22p4
C 1s22s22p5 1s22s22p6
D 1s22s22p6 1s22s22p63s1
Mark scheme
Show the mark scheme
12 C (AO1) 1 2 2 5 2 2 6
C 1s 2s 2p 1s 2s 2p
How to answer it
First Ionisation Energy Across Period 2
This question evaluates your understanding of Period 2 periodic trends in first ionisation energy (IE₁), specifically requiring you to:
- Identify elements from their full electron configurations.
- Recall the general increase in IE₁ across a period due to increasing nuclear charge and similar shielding.
- Recognise the two classic exceptions in Period 2 (Group 2 to Group 3, and Group 5 to Group 6).
- Account for the sharp decrease in IE₁ when beginning a new quantum shell.
Question 12 Breakdown
Identifying the pair where IE₁(Y) > IE₁(X)
✅ Correct Answer: C
Option C:
Atom X: 1s²2s²2p⁵ (Fluorine, F)
Atom Y: 1s²2s²2p⁶ (Neon, Ne)
Neon has a higher first ionisation energy than Fluorine because it follows the standard general trend across Period 2: Neon has an extra proton (+10 vs +9), identical shielding (both have outer electrons in the second shell), and a smaller atomic radius. Therefore, the outermost electron is held more tightly.
🧠 Exam Technique: Decode the Elements First
Count the electrons or look at the valence subshell to immediately write down the identity of each element:
- A: X = Be (Z=4), Y = B (Z=5)
- B: X = N (Z=7), Y = O (Z=8)
- C: X = F (Z=9), Y = Ne (Z=10)
- D: X = Ne (Z=10), Y = Na (Z=11)
Once named, you can spot the anomalies instantly without getting overwhelmed by configurations.
📐 Option-by-Option Analysis
| Pair | Identity of X & Y | Comparison of First IE | Why? |
|---|---|---|---|
| A | X = Beryllium (Be) Y = Boron (B) | IE₁(Y) < IE₁(X) (IE drops) | Boron's outer electron is in a higher-energy 2p subshell, which is further from the nucleus and partially shielded by the 2s² pair. Less energy is needed to remove it. |
| B | X = Nitrogen (N) Y = Oxygen (O) | IE₁(Y) < IE₁(X) (IE drops) | Oxygen has a paired electron in one of its 2p orbitals ( 2p⁴ ). Inter-electron spin-pair repulsion makes this electron easier to remove than Nitrogen's singly occupied orbitals ( 2p³ ). |
| C | X = Fluorine (F) Y = Neon (Ne) | IE₁(Y) > IE₁(X) (IE rises) | Normal periodic trend. Both have electrons removed from 2p. Neon has greater nuclear charge (+10 vs +9) with similar shielding, so attraction is stronger. Correct! |
| D | X = Neon (Ne) Y = Sodium (Na) | IE₁(Y) << IE₁(X) (Huge drop) | Sodium’s outer electron occupies the 3s orbital in a new shell. It experiences significantly increased shielding from inner shells and is much further from the nucleus. |
💡 Key Knowledge: The Two Period 2 Drops
Across Period 2 (Li to Ne), first ionisation energy generally increases, but there are two specific dips you must memorise:
- Be → B (Group 2 → 3): Drop due to subshell change (from 2s to higher-energy 2p).
- N → O (Group 5 → 6): Drop due to mutual repulsion between paired electrons in the same 2p orbital.
- Ne → Na (Period 2 → 3): Massive drop due to entering a new quantum shell (3s), with vastly increased shielding and radius.
❌ Common Student Errors
- Assuming IE always increases: Students forget the anomalies at Boron (Option A) and Oxygen (Option B), incorrectly assuming higher proton number always means higher IE.
- Confusing the question requirement: Forgetting the inequality asked for IE(Y) > IE(X) and accidentally choosing an anomaly where IE decreases.
- Assuming noble gases always have lower IE: Believing noble gases are unreactive and therefore don't follow trends, rather than having the highest IE in their period.
Topics
Physical Chemistry · 3.1.1 Atomic Structure
Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.