Explain what is meant by an isotope and describe how two isotopes of the same element are similar and how they differ.

AQA A-Level Physics (7408) — 3.2.1 Particles · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. They therefore have the same atomic number and the same number of electrons, so they exhibit the same chemical properties. Because the number of neutrons differs, the mass numbers of the isotopes are different, giving them different atomic masses.

Examiner tips

Use the definition first, then list similarity (same Z, same electrons) and difference (neutrons, mass number).
Show the link between electrons and chemical behaviour.
Keep the answer concise – 5 marks can be earned with 3–4 short sentences.

Common mistakes

Confusing isotopes with ions (changing electron number).
Forgetting to mention the same number of protons or electrons.

Mark scheme (5 marks)

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons
Isotopes have different numbers of neutrons (in the nucleus)
Isotopes have the same number of electrons
Isotopes have the same chemical properties (because chemical properties depend on the number of electrons)
Isotopes have different mass numbers (because they have different numbers of neutrons, so the total number of particles in the nucleus differs)

Key terms in this question

isotope