Two atoms of the same element have different nucleon numbers. State what is meant by the term 'nucleon number' and explain why two atoms of the same element can have different nucleon numbers.

Cambridge International IGCSE Physics (0625) — 5.1 The nuclear model of the atom · State and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Nucleon number is the total number of protons and neutrons in an atom’s nucleus.

Atoms of the same element have the same number of protons, but they can have different numbers of neutrons. Therefore the total number of nucleons (protons + neutrons) can differ. These atoms are called isotopes of the same element.

Examiner tips

  • Use the exact definition of nucleon number – protons plus neutrons. Show that the proton number is the same for the element. Explain that the difference comes from a different neutron count. Mention the term "isotopes" to link the idea to the question.

Common mistakes

  • Confusing nucleon number with atomic number. Forgetting to state that the proton number is unchanged. Not mentioning that the atoms are isotopes.

Mark scheme (4 marks)

  1. Nucleon number is the total number of protons and neutrons in an atom / nucleus.
  2. Atoms of the same element have the same number of protons / the same proton number.
  3. The two atoms have different numbers of neutrons.
  4. These atoms are called isotopes (of the same element).

Key terms in this question

nucleon number

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