Phosphorus has atomic number 15. An atom of the most common isotope of phosphorus has a mass number of 31. State the number of protons, electrons and neutrons in this atom, and explain how the number of electrons is related to the atomic number.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.2 Atoms · State · 4 marks · View as Markdown

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

Model answer (4 marks)

Number of protons: 15
Number of electrons: 15
Number of neutrons: 16
The number of electrons equals the atomic number because the atom is electrically neutral, so the number of electrons equals the number of protons.

Examiner tips

  • Use the word ‘Number of’ for each part to show you are answering the ‘State’ command.
  • Show the calculation for neutrons: 31 – 15 = 16.
  • Explain the electron–atomic number relationship in one clear sentence.
  • Use UK spelling (e.g. ‘neutrons’).

Common mistakes

  • Writing 17 neutrons instead of 16 (mis‑subtracting).
  • Confusing the atomic number with the mass number for electrons.
  • Omitting the explanation of the electron–atomic number link.

Mark scheme (4 marks)

  1. Number of protons = 15
  2. Number of electrons = 15
  3. Number of neutrons = 16 (mass number minus atomic number, i.e. 31 − 15)
  4. The number of electrons equals the atomic number / the number of protons, because atoms are electrically neutral / have no overall charge

Key terms in this question

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

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