Describe the structure of an atom of magnesium-24 and an atom of magnesium-25. Include the number of each subatomic particle in both atoms and explain how the two atoms are related to each other.

AQA GCSE Physics (8463) — 4.4.1 Atoms and isotopes · Describe · 4 marks · View as Markdown

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

Magnesium-24 has an atomic number of 12 and a mass number of 24. Magnesium-25 has an atomic number of 12 and a mass number of 25.

Model answer (4 marks)

Magnesium‑24:
- 12 protons
- 12 neutrons
- 12 electrons

Magnesium‑25:
- 12 protons
- 13 neutrons
- 12 electrons

Both atoms have the same number of protons (atomic number 12). They are isotopes of magnesium because they have the same number of protons but different numbers of neutrons, giving different mass numbers (24 and 25).

Examiner tips

  • Use the word ‘isotopes’ to link the two atoms
  • Show the sub‑particle counts clearly
  • Mention the same atomic number and different neutron numbers
  • Keep the answer concise and to the point

Common mistakes

  • Confusing neutrons with protons or electrons
  • Omitting the definition of isotopes
  • Writing the mass numbers incorrectly

Mark scheme (4 marks)

  1. Magnesium-24 has 12 protons, 12 neutrons and 12 electrons
  2. Magnesium-25 has 12 protons, 13 neutrons and 12 electrons
  3. Both atoms have the same number of protons (atomic number)
  4. They are isotopes because they have the same number of protons but different numbers of neutrons (different mass numbers)

Related

More Atoms and isotopes questions

▶ Try answering this question with AI marking (free) →