Carbon-12 and carbon-14 are both atoms of the element carbon. Explain why these two atoms are described as isotopes of carbon, and state how the number of neutrons in each atom can be determined from nuclide notation.

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

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

Carbon-12 has a proton number of 6 and a nucleon number of 12. Carbon-14 has a proton number of 6 and a nucleon number of 14.

Model answer (4 marks)

Both atoms have the same proton number (6) so they are the same element. They differ in nucleon number (12 vs 14), meaning they have different numbers of neutrons. The number of neutrons is found by subtracting the proton number from the nucleon number. Thus carbon‑12 has 12−6=6 neutrons and carbon‑14 has 14−6=8 neutrons.

Examiner tips

  • Use the term ‘isotopes’ to link same element, different mass number.
  • Show the calculation 12−6=6 and 14−6=8 to earn the full marks.
  • Mention the proton number explicitly to satisfy the first point.

Common mistakes

  • Confusing nucleon number with mass number; students write ‘mass number’ instead of ‘nucleon number’.
  • Failing to calculate the neutron numbers or giving the wrong values.
  • Using the word ‘atoms’ instead of ‘isotopes’ in the first sentence.

Mark scheme (4 marks)

  1. Both atoms have the same number of protons (proton number = 6)
  2. They have different numbers of neutrons / different nucleon numbers
  3. Number of neutrons is found by subtracting the proton number from the nucleon number
  4. Correct neutron numbers stated for both: carbon-12 has 6 neutrons and carbon-14 has 8 neutrons

Key terms in this question

isotopes · neutrons · nuclide notation

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