A student reacts calcium with chlorine gas. Calcium is in Group 2 of the periodic table and chlorine is in Group 7. Explain why calcium and chlorine form ions, state the charges on the ions formed, and explain how these charges determine the formula of calcium chloride.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Calcium is in Group 2 and has two valence electrons. It loses these two electrons to achieve a noble‑gas configuration, forming a Ca²⁺ ion.
Chlorine is in Group 7 and has seven valence electrons. It gains one electron to complete its octet, forming a Cl⁻ ion.
Ions are formed so that each atom attains a stable electron arrangement.
The charges on the ions must balance so that the overall charge of the compound is zero.
A Ca²⁺ ion requires two Cl⁻ ions to balance its charge, giving the formula CaCl₂.
Chlorine is in Group 7 and has seven valence electrons. It gains one electron to complete its octet, forming a Cl⁻ ion.
Ions are formed so that each atom attains a stable electron arrangement.
The charges on the ions must balance so that the overall charge of the compound is zero.
A Ca²⁺ ion requires two Cl⁻ ions to balance its charge, giving the formula CaCl₂.
Examiner tips
- Use the command word ‘explain’ – give reasons for ion formation and charge balance. Show the electron transfer for Ca and Cl. State the charges explicitly and then show how they combine to give CaCl₂. Keep the answer concise – 5 marks allow a short paragraph or numbered points.
Common mistakes
- Writing Ca⁺ or Cl⁺ instead of Ca²⁺ and Cl⁻. Forgetting that two Cl⁻ ions are needed to balance one Ca²⁺ ion. Not explaining why the ions form (stable octet).
Mark scheme (5 marks)
- Calcium loses two electrons to form a Ca²⁺ ion (because it is in Group 2) / atoms lose or gain electrons to form ions
- Chlorine gains one electron to form a Cl⁻ ion (because it is in Group 7)
- Atoms form ions to achieve a full outer shell of 8 electrons / to gain a stable electron arrangement
- The charges on the ions must balance / the overall charge of the compound must be zero
- Two Cl⁻ ions are needed to balance one Ca²⁺ ion, giving the formula CaCl₂
Key terms in this question
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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