Sodium has an atomic number of 11 and a mass number of 23. State the number of protons, neutrons and electrons in a sodium atom. Then explain why an atom of sodium has an overall charge of zero.

WJEC A-Level Chemistry (Wales) — 2.1 Periodic table · State and Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Number of protons: 11
Number of neutrons: 12
Number of electrons: 11
The number of negatively charged electrons equals the number of positively charged protons, so the positive and negative charges cancel out, giving an overall charge of zero.

Examiner tips

  • Use the exact numbers for protons, neutrons and electrons. Show the equality of electrons and protons to justify the zero charge. Keep the explanation concise and use the phrase ‘cancel out’ or ‘equal and opposite’.
  • common_mistakes
  • :
  • Writing 13 neutrons instead of 12. Forgetting to state that the charges cancel out. Using vague terms like ‘neutral’ without explaining the charge balance.

Mark scheme (5 marks)

  1. Number of protons = 11
  2. Number of neutrons = 12
  3. Number of electrons = 11
  4. The number of (negatively charged) electrons equals the number of (positively charged) protons
  5. The positive and negative charges cancel out / are equal and opposite, giving an overall charge of zero

Key terms in this question

atomic number · mass number · proton · neutron · electron

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