Explain how the relative masses and relative charges of the subatomic particles in an atom account for the overall mass and charge of the atom.

Edexcel A-Level Chemistry (9CH0) — 1.1 Atomic structure · Explain · 5 marks · View as Markdown

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

An atom of sodium has 11 protons, 12 neutrons and 11 electrons.

Model answer (5 marks)

Protons and neutrons each have a relative mass of 1, so the mass of a sodium atom is the sum of 11 protons and 12 neutrons, giving a mass number of 23.
Electrons have a negligible relative mass (≈0), so they do not add to the mass.
Protons carry a charge of +1, electrons –1, neutrons 0. The atom has 11 protons and 11 electrons, so the +1 and –1 charges cancel and the overall charge is zero.

Examiner tips

  • Mention the relative mass of protons/neutrons and the negligible mass of electrons. State the mass number calculation. Explain the charge of each particle and the cancellation of charges. Keep the answer concise and use the exact terminology.
  • common_mistakes
  • :
  • Saying electrons contribute to mass. Forgetting that neutrons have no charge. Not recognising that the atom is neutral because protons = electrons.

Mark scheme (5 marks)

  1. Protons and neutrons are both assigned a relative mass of 1 (each), so they contribute to the total mass of the atom
  2. Electrons have a relative mass of approximately 0 (or negligible/very small), so they do not contribute significantly to the mass of the atom
  3. The mass number (or relative atomic mass) equals the number of protons plus the number of neutrons, giving sodium a mass of 23
  4. Protons have a relative charge of +1 and electrons have a relative charge of −1; neutrons have no charge (relative charge of 0)
  5. The number of protons equals the number of electrons, so the positive and negative charges cancel out, meaning the atom has no overall charge

Key terms in this question

relative mass · relative charge

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