Oxygen has an atomic number of 8 and a mass number of 16. State the number of protons, neutrons and electrons in one atom of oxygen, and explain why the overall charge of the atom is zero.

WJEC GCSE Chemistry (Wales) — 2.1 Bonding, structure and properties · 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: 8
Neutrons: 8
Electrons: 8
The atom is neutral because the 8 negatively charged electrons balance the 8 positively charged protons, giving an overall charge of zero.

Examiner tips

  • Use the exact numbers for each particle; examiners expect the count, not a description of the process.
  • State the charge balance explicitly – mention electrons = protons → zero charge.
  • Keep the answer concise; no extra wording.

Common mistakes

  • Confusing mass number with atomic number; writing 16 protons instead of 8.
  • Forgetting to mention that the charges cancel, or stating only the numbers without the charge explanation.

Mark scheme (4 marks)

  1. Number of protons = 8
  2. Number of neutrons = 8 (mass number − atomic number = 16 − 8)
  3. Number of electrons = 8
  4. The number of (negatively charged) electrons equals the number of (positively charged) protons, so the charges cancel / balance out

Key terms in this question

atomic number · mass number · proton · neutron · electron

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