Phosphorus has atomic number 15 and mass number 31. State the number of protons, neutrons and electrons in one atom of phosphorus, and state in which electron shells these electrons are arranged.

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

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

Model answer (4 marks)

Protons: 15
Neutrons: 16
Electrons: 15
Electron shells: 2, 8, 5

Examiner tips

  • Use the exact numbers given in the mark scheme; avoid rounding or approximations.
  • Show the electron shell arrangement as a sequence of numbers separated by commas.
  • Remember the command word "State" – give the facts directly, no extra explanation.
  • Use UK spelling (e.g., "electrons").

Common mistakes

  • Writing 14 protons instead of 15; confusion between atomic number and mass number.
  • Listing shells as 2, 8, 3 or 2, 8, 4 – mis‑counting valence electrons.
  • Using a comma‑separated list but including spaces or extra punctuation that may be penalised.

Mark scheme (4 marks)

  1. Number of protons = 15
  2. Number of neutrons = 16
  3. Number of electrons = 15
  4. Correct electron shell arrangement: 2, 8, 5

Key terms in this question

atomic number · mass number · proton · neutron · electron · electron shell

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