An atom of carbon-12 and an atom of carbon-14 are both described as isotopes of carbon. State what is meant by the term isotope, and explain why carbon-12 and carbon-14 have different mass numbers despite being the same element.

Eduqas GCSE Physics — 9.1 Nuclear atom and isotopes · 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)

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Carbon‑12 has 6 protons and 6 neutrons, giving a mass number of 12.
Carbon‑14 has 6 protons and 8 neutrons, giving a mass number of 14.

Because the two isotopes have a different number of neutrons, their total number of protons plus neutrons differs, so their mass numbers are different.

Examiner tips

  • Use the definition of isotope first, then give the specific neutron counts for C‑12 and C‑14.
  • Show the calculation of mass number as protons + neutrons for each isotope.

Common mistakes

  • Confusing mass number with atomic mass or atomic weight.
  • Saying the isotopes have different numbers of protons instead of neutrons.

Mark scheme (4 marks)

  1. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons
  2. The mass number is the total number of protons and neutrons (nucleons) in the nucleus
  3. Carbon-12 has 6 neutrons whereas carbon-14 has 8 neutrons (both have 6 protons)
  4. Because carbon-14 has more neutrons, the total number of protons plus neutrons is greater, giving it a higher mass number

Key terms in this question

isotope · mass number

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