Calcium has atomic number 20 and mass number 40. Describe the structure of a calcium atom, including the arrangement of its subatomic particles and the distribution of its electrons in shells.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.2 Atoms · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

The nucleus contains 20 protons, giving the atom a positive charge. It also contains 20 neutrons (40 − 20). The atom has 20 electrons that orbit the nucleus and carry a negative charge. The electrons are arranged in shells: 2 in the first shell, 8 in the second, 8 in the third and 2 in the outermost (fourth) shell.

Examiner tips

  • Use the exact terms ‘protons’, ‘neutrons’, ‘electrons’, ‘nucleus’, ‘shells’
  • Show the electron configuration 2,8,8,2
  • Mention the charge of each particle

Common mistakes

  • Confusing the number of neutrons with the mass number
  • Omitting the outermost shell or giving the wrong electron count

Mark scheme (4 marks)

  1. The nucleus contains 20 protons (and is positively charged)
  2. The nucleus contains 20 neutrons (mass number minus atomic number = 40 − 20)
  3. The atom has 20 electrons which orbit/surround the nucleus (negatively charged)
  4. Electrons are arranged in shells as 2, 8, 8, 2 (outermost/fourth shell contains 2 electrons)

Key terms in this question

atomic number · mass number · electron

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