Calcium chloride is an ionic compound formed when calcium reacts with chlorine. Explain how calcium chloride is formed from its elements, including how the ions are formed and why the compound has the formula CaCl₂.

Edexcel A-Level Chemistry (9CH0) — 2.1 Ionic bonding · Explain · 5 marks · View as Markdown

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

Calcium is in Group 2 of the periodic table. Chlorine is in Group 7 of the periodic table.

Model answer (5 marks)

Calcium (Ca) has two valence electrons. When it reacts it loses these two electrons to give a Ca²⁺ ion.

Chlorine (Cl) has seven valence electrons. Each Cl atom gains one electron to reach eight electrons and becomes a Cl⁻ ion.

The electrons are transferred from Ca to Cl; they are not shared, so the bond is ionic.

Both Ca²⁺ and Cl⁻ ions achieve a full outer shell (stable electronic configuration) after the transfer.

Because one Ca²⁺ ion needs two Cl⁻ ions to balance the charge, the empirical formula is CaCl₂.

Examiner tips

  • Use the word ‘lose’ for Ca and ‘gain’ for Cl to show electron transfer. Show the charge balance (2+ = 2×1−) to justify CaCl₂. Mention the stable octet to explain why the ions form.
  • common_mistakes
  • :
  • Saying Ca loses one electron or Cl gains two electrons. Forgetting to state that the bond is ionic. Writing CaCl instead of CaCl₂.

Mark scheme (5 marks)

  1. Calcium atom loses 2 electrons to form a Ca²⁺ ion
  2. Each chlorine atom gains 1 electron to form a Cl⁻ ion
  3. Electrons are transferred from calcium to chlorine (not shared)
  4. Both ions achieve a full outer shell / stable electronic configuration
  5. Two chloride ions are needed to accept the two electrons from one calcium atom, so the formula is CaCl₂

Key terms in this question

ionic compound · ion · formula

Related

More Ionic bonding questions

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