An atom of uranium-235 has an atomic number of 92. State the number of protons, neutrons, and electrons in a neutral atom of uranium-235, and explain how you determined the number of neutrons.

Eduqas GCSE Physics — 9.1 Nuclear atom and isotopes · State · 4 marks · View as Markdown

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

Model answer (4 marks)

Protons: 92
Neutrons: 143
Electrons: 92
The neutron number is found by subtracting the atomic number (92) from the mass number (235).

Examiner tips

  • Use the exact wording: 'Protons: 92', 'Neutrons: 143', 'Electrons: 92'.
  • Show the calculation: 235 – 92 = 143.
  • Remember the atom is neutral, so electrons = protons.
  • Keep the answer concise and to the point.

Common mistakes

  • Writing the wrong neutron number (e.g. 141 or 144).
  • Forgetting that a neutral atom has equal numbers of protons and electrons.
  • Not showing the subtraction to justify the neutron count.

Mark scheme (4 marks)

  1. Number of protons = 92
  2. Number of electrons = 92 (in a neutral atom)
  3. Number of neutrons = 143
  4. Neutron number is found by subtracting the atomic number (proton number) from the mass number

Key terms in this question

atomic number · proton · neutron · electron · neutral atom

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