OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 6

1 mark · Medium difficulty · Multiple Choice

Deduce the formula of the oxide of a Period 3 element using its successive ionisation energies.

Practise this question

Question

A multiple-choice question showing a table of successive ionisation energies for a Period 3 element from the 1st to the 9th ionisation energy. The values jump significantly between the 3rd and 4th ionisation energies, going from 2745 to 11578 kJ mol-1. Four options are given: A (Na2O), B (MgO), C (Al2O3), and D (SiO2), with a box to write the final answer.
Question text

6 Successive ionisation energies, in kJ mol–1, of an element in Period 3 of the periodic table are

shown below.

1st 2nd 3rd 4th 5th 6th 7th 8th 9th

578 1817 2745 11578 14831 18378 23296 27460 31862

What is the formula of the oxide of the Period 3 element?

A Na2O

B MgO

C Al2O3

D SiO2

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 6 is option C.

6 C 1 AO2.1

How to answer it

Determining Period 3 Element Oxide Formula from Successive Ionisation Energies

What this question tests

This question assesses your ability to interpret successive ionisation energy data to deduce the group number (and hence identity) of a Period 3 element, and subsequently apply valency rules to determine the correct formula of its oxide.

Question Analysis & Answer

Multiple Choice Question 6

✅ Correct Answer: C (Al₂O₃)

The element has 3 electrons in its outer shell (Group 13 / III), which is aluminium (Al). Aluminium forms an Al³⁺ ion and combines with oxide ions (O²⁻) to give the formula Al₂O₃ .

💡 Key Knowledge

  • Group Identification: A large jump in successive ionisation energy indicates an electron being removed from a new, inner quantum shell closer to the nucleus.
  • Counting Valence Electrons: Count how many electrons are removed before the huge jump occurs. This equals the group number.
  • Period 3 Valencies: Na (Group 1, Na₂O), Mg (Group 2, MgO), Al (Group 13/III, Al₂O₃), Si (Group 14/IV, SiO₂).

🧠 Exam Technique

Scan the data table horizontally from left to right looking for the massive numerical leap. Here, the jump happens between the 3rd ( 2745 ) and 4th ( 11578 ) ionisation energies. This tells you there are 3 valence electrons.

❌ Common Errors

  • Miscounting the number of electrons before the jump and choosing the wrong group.
  • Correctly identifying the element as aluminium (Al) but guessing the formula of aluminium oxide as AlO or AlO₂ instead of balancing the ionic charges ( Al³⁺ and O²⁻ ).
Mark Scheme Allocation: 1 mark for option C (AO2.1 - Applying knowledge and understanding of ionisation energies to an unfamiliar context).

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.2 Electrons, bonding and structure · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.