Magnesium and calcium are both elements in Group 2 of the periodic table. When each element reacts with cold water, magnesium reacts very slowly whilst calcium reacts more vigorously, producing a metal hydroxide and hydrogen gas. Explain why calcium is more reactive than magnesium, and describe one similarity you would expect in the products formed when each metal reacts with water.

WJEC A-Level Chemistry (Wales) — 2.2 Group 1, 2 and 7 · Explain · 5 marks · View as Markdown

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

Magnesium and calcium are both in Group 2 of the periodic table. Magnesium has atomic number 12 and calcium has atomic number 20.

Model answer (5 marks)

Calcium has more electron shells than magnesium, so its outermost electron is further from the nucleus. The greater distance reduces the effective nuclear charge felt by this electron, making it easier to remove. Consequently calcium loses its outer electron more readily than magnesium, giving it a higher reactivity with cold water.
Both metals react with water to give a metal hydroxide and hydrogen gas.

Examiner tips

  • Use the word ‘reactive’ and explain the role of electron shells and effective nuclear charge; mention the products explicitly.
  • Show the step‑by‑step reasoning: more shells → outer electron further → easier loss → higher reactivity.
  • Include the similarity in products to gain the full marks for the second part.

Common mistakes

  • Confusing ‘more reactive’ with ‘faster reaction rate’ without explaining electron structure.
  • Failing to state that both reactions produce a metal hydroxide and hydrogen gas.
  • Using vague terms like ‘easier’ without linking to electron shell distance or nuclear charge.

Mark scheme (5 marks)

  1. Calcium has more electron shells / energy levels than magnesium
  2. The outer electron in calcium is further from the nucleus than in magnesium
  3. The outer electrons in calcium are lost more easily (than in magnesium)
  4. Both metals produce a metal hydroxide (as one of the products)
  5. Both metals produce hydrogen gas (as one of the products)

Key terms in this question

Group 2 · metal hydroxide

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