Magnesium has atomic number 12. An atom of magnesium has two electrons in its outermost shell. Describe the arrangement of electrons in a magnesium atom and explain how the atomic number relates to the number of electrons in a neutral magnesium atom.

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).

Magnesium is a metal in Period 3 of the periodic table. It has atomic number 12 and its most common isotope has a mass number of 24.

Model answer (4 marks)

A neutral magnesium atom has 12 electrons, equal to its 12 protons (atomic number). The electrons are arranged in shells around the nucleus. The configuration is 2 electrons in the first shell, 8 in the second, and 2 in the third (outermost) shell – written as 2, 8, 2. In a neutral atom the number of electrons equals the number of protons, so the atomic number tells us the electron count.

Examiner tips

  • State the electron count equals the atomic number; give the shell configuration 2, 8, 2; explain shells are energy levels; link atomic number to protons and electrons.

Common mistakes

  • Writing the wrong shell configuration (e.g., 2, 8, 8); confusing atomic mass with atomic number; forgetting to explain the equality of electrons and protons in a neutral atom.

Mark scheme (4 marks)

  1. A neutral magnesium atom has 12 electrons (equal to the number of protons / atomic number)
  2. Electrons are arranged in shells/energy levels around the nucleus
  3. The electron arrangement is 2, 8, 2 (first shell has 2, second shell has 8, third/outermost shell has 2)
  4. In a neutral atom the number of electrons equals the number of protons / the atomic number gives the number of protons, which equals the number of electrons in a neutral atom

Key terms in this question

atomic number · electron

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