AQA A-Level Chemistry AS Paper 1, June 2025: Question 14

AQA A-Level Chemistry • Paper 1 Multiple Choice

Identifying a Period 3 Element from Successive Ionisation Energies

What this question tests

This question evaluates your ability to:

Interpret patterns in successive ionisation energy data.
Identify the "large jump" corresponding to removal of an electron from an inner quantum shell.
Deduce the group number of an unknown element and link it directly to its position in Period 3 of the Periodic Table.

Question 14 Breakdown

Data Analysis & Element Identification [1 Mark]

Question: Which Period 3 element has these successive ionisation energies?

Ionisation energy	1st	2nd	3rd	4th	5th	6th
Value / kJ mol⁻¹	786	1580	3230	4360	16 000	20 000

Options: A: Al | B: Si | C: P | D: S

✅ Correct Answer

Option B (Silicon, Si) [1 mark, AO2]

The fifth ionisation energy shows a dramatic increase (from 4360 to 16 000 kJ mol⁻¹), proving that the 5th electron is removed from a shell closer to the nucleus (n = 2). Therefore, there are 4 outer-shell electrons, placing the element in Group 4 (Group 14).

📐 Step-by-Step Deduction

Calculate or observe differences:
• 1st to 2nd: +794 kJ mol⁻¹
• 2nd to 3rd: +1650 kJ mol⁻¹
• 3rd to 4th: +1130 kJ mol⁻¹
• 4th to 5th: +11 640 kJ mol⁻¹ (Nearly a 4× jump!)
Determine outer shell capacity: 4 electrons are removed relatively easily before entering a new, inner principal shell.
Find the element: Period 3 with 4 outer electrons = [Ne] 3s² 3p² = Silicon (Si).

💡 Key Knowledge

Definition: nth ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of (n-1)+ gaseous ions:
X⁴⁺(g) → X⁵⁺(g) + e⁻ (for 5th IE).
Why the jump occurs: The 5th electron is removed from the 2p subshell, which is:
• In a lower principal quantum shell ( n = 2 ).
• Closer to the nucleus.
• Experiences significantly less shielding.

🧠 Exam Technique: The "N - 1" Rule

To find the group number instantly in MCQs:

Find the ionisation energy number where the big jump starts. Here, it is the 5th.
Number of valence electrons = jump number - 1 = 5 - 1 = 4 valence electrons.
4 valence electrons = Group 4 (or 14) element → Si.

❌ Common Misconceptions & Traps

Off-by-one counting error: Students often mistakenly choose Group 5 (Phosphorus, Option C) because the jump is to the 5th value. Always count how many electrons are removed before the jump occurs.
Confusing subshell jumps with shell jumps: While there are small rises between s and p subshells (e.g. 2nd to 3rd), a shell change creates an order-of-magnitude leap (thousands of kJ mol⁻¹ increase, often 3× to 5× the preceding value).
Confusing Period with Group: Ensure you check both the period stated in the stem (Period 3) and the group deduced from the table.