A neutral atom of sodium has a proton number of 11 and a nucleon number of 23. State the number of protons, neutrons, and electrons in this atom, and explain how the atom would change if it lost one electron.

Cambridge International IGCSE Physics (0625) — 5.1 The nuclear model of the atom · 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: 11
Neutrons: 12 (23−11)
Electrons: 11
If one electron is lost the atom becomes a +1 ion – there are now 11 protons and 10 electrons, so the charge is +1.

Examiner tips

  • Use the exact numbers for each particle; show the neutron calculation. Explain the charge change in one sentence. Use the term ‘positive ion’ or ‘cation’ as expected.

Common mistakes

  • Saying the atom becomes neutral after losing an electron. Giving the wrong number of neutrons (e.g. 13). Not stating that the ion is +1 or using ‘negative ion’.

Mark scheme (4 marks)

  1. 11 protons
  2. 12 neutrons (nucleon number minus proton number = 23 − 11)
  3. 11 electrons (equal to number of protons in a neutral atom)
  4. Losing one electron would form a positive ion / the atom would become a positive ion because there would be more protons than electrons

Key terms in this question

proton number · nucleon number · neutrons · electrons

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