Magnesium has atomic number 12. Describe the structure of a magnesium atom, including the number and location of each type of subatomic particle and the arrangement of its electrons in shells.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.3 Atomic structure · Describe · 4 marks · View as Markdown

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

Magnesium is a metal in Group 2 of the periodic table. It has atomic number 12 and mass number 24.

Model answer (4 marks)

The nucleus contains 12 protons and, because the mass number is 24, 12 neutrons. The atom has 12 electrons orbiting the nucleus. The electrons are arranged in shells: 2 electrons in the first shell (n=1) and 8 electrons in the second shell (n=2). The remaining 2 electrons occupy the third (outermost) shell (n=3).

Examiner tips

  • Use the exact numbers of protons, neutrons and electrons; include the mass number to justify neutrons.
  • State the shell numbers (n=1, n=2, n=3) and the electron count in each.
  • Mention that the third shell is the outermost.
  • Keep the answer concise and in a single paragraph or numbered points.

Common mistakes

  • Failing to mention the 12 neutrons or mis‑counting them.
  • Omitting the third shell or giving the wrong electron distribution.
  • Using vague terms like "outer shell" without specifying n=3.

Mark scheme (4 marks)

  1. The nucleus contains 12 protons (and 12 neutrons for mass number 24)
  2. The atom has 12 electrons orbiting the nucleus
  3. Electrons are arranged in shells/energy levels: 2 electrons in the first shell and 8 in the second shell
  4. 2 electrons in the third (outermost) shell

Key terms in this question

atomic number · electron

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