Explain how the atomic number and mass number of an atom can be used to determine the number of protons, neutrons and electrons in that atom. Use the example of an atom with atomic number 11 and mass number 23.

Eduqas GCSE Chemistry — C1 Atomic structure and the periodic table · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The atomic number equals the number of protons – here 11 protons.
In a neutral atom the number of electrons equals the number of protons – 11 electrons.
The number of neutrons is found by subtracting the atomic number from the mass number: 23 – 11 = 12 neutrons.
Thus the atom has 11 protons, 11 electrons and 12 neutrons.

Examiner tips

  • Write the atomic number first to show protons, then state electrons for a neutral atom, finally calculate neutrons by subtraction.
  • Use the example numbers to demonstrate each step clearly.

Common mistakes

  • Confusing mass number with atomic number, or vice versa.
  • Forgetting that a neutral atom has equal numbers of protons and electrons.

Mark scheme (4 marks)

  1. The atomic number equals the number of protons (in this case 11 protons)
  2. In a neutral atom the number of electrons equals the number of protons (so 11 electrons)
  3. The number of neutrons is found by subtracting the atomic number from the mass number
  4. The number of neutrons in this atom is 12

Key terms in this question

atomic number · mass number · proton · neutron · electron

Related

More Atomic structure and the periodic table questions

▶ Try answering this question with AI marking (free) →