Explain what is meant by the term 'isotopes' and describe how two isotopes of the same element are both similar and different in terms of their subatomic particles.

Edexcel GCSE Chemistry (1CH0) — 1.3 Atomic structure · 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.

Both isotopes have the same number of protons, and therefore the same number of electrons, so they have the same chemical properties.

They differ in the number of neutrons; one isotope has more neutrons than the other.

Because the neutron number differs, the isotopes have different mass numbers (and therefore different relative atomic masses).

Examiner tips

  • State that isotopes share the same proton number and differ in neutron number; link this to identical chemistry but different mass.
  • Use the phrase ‘same number of protons’ and ‘different number of neutrons’ to hit the key points.
  • Mention that the difference in neutrons gives different mass numbers to show full understanding.

Common mistakes

  • Confusing isotopes with ions (changing electron number).
  • Omitting that the chemical properties are the same because proton number is unchanged.
  • Failing to mention the difference in mass numbers caused by neutron variation.

Mark scheme (4 marks)

  1. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
  2. Two isotopes have the same number of protons (and the same number of electrons).
  3. Two isotopes have different numbers of neutrons.
  4. The difference in the number of neutrons means isotopes have different mass numbers (relative atomic masses).

Key terms in this question

isotopes

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