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

Eduqas A-Level Chemistry — 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).

Sodium is an element in Group 1 of the periodic table.

Model answer (5 marks)

Protons: 11
Neutrons: 12
Electrons: 11
The protons carry a +1 charge and the electrons a –1 charge. Because the numbers of protons and electrons are equal, their charges cancel, giving the atom an overall charge of zero.

Examiner tips

  • Use the exact numbers (11, 12, 11) to match the mark scheme. Explain that charges are equal and opposite. Show the cancellation to justify the zero charge.

Common mistakes

  • Writing 13 neutrons instead of 12. Forgetting to state that the charges cancel. Using the wrong sign for electron charge.

Mark scheme (5 marks)

  1. Number of protons = 11
  2. Number of neutrons = 12 (mass number minus atomic number: 23 − 11)
  3. Number of electrons = 11
  4. Protons are positively charged and electrons are negatively charged (accept: equal and opposite charges)
  5. The number of protons equals the number of electrons, so the charges cancel / balance out, giving an overall charge of zero

Key terms in this question

atomic number · mass number · proton · neutron · electron

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