Explain why two atoms of the same element can have different mass numbers, and describe how this affects the relative atomic mass of that element.

WJEC A-Level Chemistry (Wales) — 2.3 Transition metals (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Atoms of the same element have the same number of protons (the atomic number) but can have different numbers of neutrons. These variants are called isotopes. The mass number of an atom is the sum of its protons and neutrons, so different neutron numbers give different mass numbers. The relative atomic mass of an element is an average of the mass numbers of its naturally occurring isotopes, weighted by their relative abundances.

Examiner tips

  • Use the word "isotopes" to show you understand the concept.
  • Explain that mass number = protons + neutrons and that different neutrons give different mass numbers.
  • Mention that the relative atomic mass is a weighted average of the isotopes’ mass numbers.
  • Keep the answer concise – 5 marks can be earned with a short paragraph.

Common mistakes

  • Confusing mass number with atomic mass.
  • Forgetting to mention that the relative atomic mass is an average weighted by abundance.
  • Using the wrong terminology (e.g., calling isotopes atoms of different elements).

Mark scheme (5 marks)

  1. Atoms of the same element always have the same number of protons (atomic number)
  2. But atoms of the same element can have a different number of neutrons
  3. These atoms are called isotopes
  4. Mass number = protons + neutrons, so different numbers of neutrons give different mass numbers
  5. Relative atomic mass is an average value that takes account of the abundance of each isotope

Key terms in this question

mass number · relative atomic mass

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