Explain why atoms are electrically neutral, and describe what happens to the overall charge of an atom when it gains or loses electrons.

WJEC A-Level Physics (Wales) — 1.7 Particles and nuclear structure · Explain · 5 marks · View as Markdown

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

Atoms contain charged particles but are normally found to have no overall electric charge. However, atoms can gain or lose electrons to form ions.

Model answer (5 marks)

Atoms are electrically neutral because the nucleus contains protons, each with a +1 charge, and the electrons orbiting the nucleus each have a –1 charge. In a neutral atom the number of protons equals the number of electrons, so the positive and negative charges cancel.

If an atom gains electrons, it has more negative charge than positive and becomes a negatively charged ion (anion). If it loses electrons, it has more positive charge than negative and becomes a positively charged ion (cation).

Examiner tips

  • Use the word ‘neutral’ and explain charge cancellation; mention protons and electrons explicitly.
  • Show the change in charge when electrons are added or removed – an anion or cation – to cover all points.
  • Keep the answer concise – 5 marks can be earned with 3–4 short sentences.

Mark scheme (5 marks)

  1. The nucleus contains protons, which have a positive charge
  2. Electrons have a negative charge (and orbit/surround the nucleus)
  3. In a neutral atom the number of protons equals the number of electrons, so the charges cancel / balance
  4. When an atom gains electrons it becomes a negatively charged ion
  5. When an atom loses electrons it becomes a positively charged ion

Key terms in this question

electrically neutral · electron

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