Edexcel A-Level Chemistry AS Paper 1, June 2019: Question 3

4 marks · Medium difficulty · Calculations

Define isotopes in terms of subatomic particles and calculate the mass number of a second gallium isotope given its relative atomic mass and the abundance of the first isotope.

Practise this question

Question

Question 3 asks about isotopes in two parts. Part (a) asks to state, in terms of subatomic particles, what is meant by the term isotopes, worth 2 marks with four blank lines for an answer. Part (b) states that the element gallium has a relative atomic mass of 69.735 and contains only two isotopes, with the isotope 69Ga having a relative abundance of 63.25%. It asks to calculate the mass number of the other isotope showing all working, worth 2 marks.
Question text

3 This question is about isotopes.

(a) State, in terms of subatomic particles, what is meant by the term isotopes.

(2)

(b) The element gallium has a relative atomic mass of 69.735 and only contains two

isotopes.

A sample of gallium contained the isotope 69Ga, with a relative abundance of 63.25 %.

Calculate the mass number of the other isotope.

You must show all your working.

(2)

(Total for Question 3 = 4 marks)

Mark scheme

Show the mark scheme Mark scheme for Question 3. For 3(a), awards 1 mark for stating same number of protons and 1 mark for different numbers of neutrons. For 3(b), awards 1 mark for correct subtraction to find the percentage/abundance of the unknown isotope (36.75%), and 1 mark for the correct calculation of the mass number (yielding 71) with suitable working.

How to answer it

Isotopes and Relative Atomic Mass Study Guide

What this question tests

This question assesses your fundamental understanding of atomic structure definitions (isotopes) and your ability to apply relative atomic mass (Ar) calculations using percentage abundances. You must demonstrate precision in subatomic particle definitions and algebraic rearrangement in multi-step calculations.

Question Part (a)

Defining Isotopes

✅ Correct Answer

  • Same number of protons (1 mark)
  • Different numbers of neutrons (1 mark)
Total: 2 marks

💡 Key Knowledge

  • Isotopes are atoms of the same element.
  • Atomic number (proton number) defines the element, so it must remain identical.
  • Mass number varies because the neutron count changes.

❌ Common Errors

  • Mentioning "same number of electrons" (this describes ions or neutral atoms, not what makes them isotopes).
  • Using vague terms like "same subatomic particles" instead of explicitly naming protons and neutrons.
Question Part (b)

Calculating the Mass Number of an Isotope

🧠 Exam Technique & Guidance

Always show every line of your working out. Even if your final integer answer slips due to a minor arithmetic error, examiners can award M1 if your initial abundance subtraction and setup are correct.

📐 Step-by-Step Calculation

  1. Find the percentage abundance of the second isotope:
    100 - 63.25 = 36.75% (or use fractions: 1.00 - 0.6325 = 0.3675 )
    (Awards Mark 1)
  2. Set up the relative atomic mass equation:
    (69 × 63.25) + (M × 36.75) / 100 = 69.735
  3. Rearrange and solve for M:
    4364.25 + 36.75M = 6973.5
    36.75M = 2609.25
    M = 71
    (Awards Mark 2)
Total: 2 marks | Note: Correct answer with no working gets M2 only. Allow Error Carried Forward (TE) from M1.

❌ Common Calculation Traps

  • Forgetting to divide the total sum by 100 when using percentages.
  • Panicking when the calculated mass number ( 71 ) is not a whole number initially, and failing to round/recognise that mass numbers must be integers.
  • Adding units (like g or g mol⁻¹ ) to the mass number — mass numbers are dimensionless integers.

Topics

Physical Chemistry · Topic 1: Atomic Structure and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.