Explain why two atoms with the same mass number can belong to different elements, and identify the subatomic property that determines which element an atom belongs to.

IB DP Chemistry Higher Level (2023 syllabus) — S1.2 The nuclear atom · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Mass number is the total of protons and neutrons in the nucleus.

Two atoms can have the same mass number yet different numbers of protons and neutrons; the different proton counts mean they are different elements.

The element is determined by the number of protons (atomic number).

Atoms with equal mass numbers but different atomic numbers are called isobars.

Examiner tips

  • Use the term ‘atomic number’ for the subatomic property.
  • Show the logic: same mass number → different Z → different element.
  • Mention isobars to demonstrate full understanding.

Common mistakes

  • Confusing mass number with atomic mass.
  • Saying ‘neutrons’ alone determine the element.
  • Forgetting to identify the property as the atomic number.

Mark scheme (4 marks)

  1. Mass number is the total number of protons and neutrons (nucleons) in the nucleus.
  2. Two atoms with the same mass number can have different numbers of protons and different numbers of neutrons that sum to the same total, so they belong to different elements.
  3. The number of protons (atomic number / proton number) determines which element an atom belongs to.
  4. Atoms that have the same mass number but different atomic numbers (and therefore belong to different elements) are called isobars.

Key terms in this question

mass number

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