AQA A-Level Chemistry AS Paper 1, November 2021: Question 25

1 mark · Medium difficulty · Multiple Choice

Identify element Z from the graph of its first seven successive ionisation energies showing a large jump after the fourth ionisation energy.

Practise this question

Question

A multiple choice question featuring a scatter graph of the first seven successive ionisation energies of element Z against ionisation number from 1 to 7, on the y-axis ranging from 0 to 25,000 kJ mol-1. The points show a gradual increase from 1 to 4, followed by a large jump between ionisation numbers 4 and 5. Below the graph, the question asks 'What is element Z?' with four options: A Carbon, B Nitrogen, C Silicon, and D Phosphorus.
Question text

25 The first seven successive ionisation energies for element Z are shown.

What is element Z?

[1 mark]

A Carbon

B Nitrogen

C Silicon

D Phosphorus

Mark scheme

Show the mark scheme The mark scheme table shows question number 25, the correct answer option C, 1 mark allocated, and the identification of Silicon.

25 C 1 Silicon

How to answer it

Identifying Elements from Successive Ionisation Energies

What this question tests

This question assesses your understanding of electronic configuration and periodic trends through successive ionisation energies. You must interpret graphical data to locate large "jumps" in energy, which correspond to removing an electron from a new, inner quantum shell closer to the nucleus.

Question 2.5 — Multiple Choice [1 Mark]

✅ Correct Answer

C Silicon

Mark Awarded: 1 / 1 for selecting option C.

💡 Key Knowledge

  • Successive ionisation energies always increase because the same nuclear charge is pulling on fewer remaining electrons.
  • A large jump in successive ionisation energy indicates that an electron is being removed from a new energy level (shell) closer to the nucleus with significantly less shielding.
  • Silicon (atomic number 14) has the electron configuration 1s² 2s² 2p⁶ 3s² 3p² .

🧠 Exam Technique

  • Count the electrons being removed up to the major jump: looking at the graph, the big jump happens between ionisation 4 and ionisation 5.
  • This means there are 4 valence electrons in the outer shell, placing element Z in Group 4.
  • Look at the given options: Carbon is in Group 4 (period 2), Silicon is in Group 4 (period 3). Since 7 ionisation energies are shown and the outer electrons are removed after the 4th, element Z must have more than 4 total electrons—ruling out carbon (total 6 electrons, but 4 outer would mean 2 inner; let's check total electrons: Si has 14, meaning 4 outer in n=3, 8 in n=2, 2 in n=1).

❌ Common Errors

  • Confusing the number of outer-shell electrons with the total number of electrons or the group number.
  • Misreading the ionisation number axis on the graph where the massive gradient change occurs.
  • Guessing between Carbon and Silicon just because both are in Group 4 without checking whether the total number of electrons matches the available data points.

Topics

Physical Chemistry · 3.1.1 Atomic Structure

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.