Explain what is meant by the term 'isotopes' and describe how two isotopes of the same element differ from each other.

AQA GCSE Physics (8463) — 4.4.1 Atoms and isotopes · 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, meaning they have the same number of protons (the same atomic number). They differ in the number of neutrons in their nuclei. Consequently, their mass numbers are different. For example, Carbon‑12 has 6 protons and 6 neutrons, while Carbon‑14 has 6 protons and 8 neutrons.

Examiner tips

Use the term ‘isotopes’ first, then state they share the same atomic number but differ in neutrons and mass number; give a clear example.
Show the example with the element name and mass numbers to hit all marks.

Common mistakes

Confusing isotopes with ions or isotones; not mentioning the same atomic number.
Giving only one difference (e.g., mass number) and omitting the neutron count or example.

Mark scheme (4 marks)

Isotopes are atoms of the same element (same number of protons / same atomic number)
Isotopes have different numbers of neutrons
Isotopes therefore have different mass numbers
Correct example given, e.g. Carbon-12 and Carbon-14 both have 6 protons but different numbers of neutrons

Key terms in this question

isotopes