# 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

> 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.

## 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)

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