Iron has atomic number 26 and mass number 56. State the number of protons, neutrons and electrons in one atom of iron. Explain why a different isotope of iron, iron-54, has the same chemical properties as iron-56.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.3 Atomic structure · State and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Protons: 26
Neutrons: 30
Electrons: 26

Iron‑54 has the same number of protons (and therefore electrons) as iron‑56, so its electron arrangement is identical. Because chemical properties are determined by the electron configuration, the two isotopes behave the same chemically.

Examiner tips

  • Use the command word ‘State’ for the numbers, then ‘Explain’ for the reasoning; mention protons/electrons first, then electron configuration.
  • Show the calculation for neutrons: 56−26=30.
  • Keep the explanation concise – focus on identical electron arrangement.

Common mistakes

  • Confusing mass number with atomic number; writing 54 neutrons instead of 30.
  • Claiming the isotopes differ chemically because of mass; ignoring that chemistry depends on electrons.
  • Including unnecessary details such as nuclear stability.

Mark scheme (4 marks)

  1. Number of protons = 26
  2. Number of neutrons = 30
  3. Number of electrons = 26
  4. Both isotopes have the same number of protons / electrons, so they have the same electron arrangement / configuration, which determines chemical properties

Key terms in this question

atomic number · mass number · isotope · electron · proton · neutron

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