Sodium chloride is an ionic compound formed when sodium reacts with chlorine. Explain how a sodium atom and a chlorine atom each form ions, and state why the charges in sodium chloride balance to give an overall neutral compound.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium is in Group 1 of the periodic table. Chlorine is in Group 7 of the periodic table.
Model answer (5 marks)
Sodium has one valence electron. It loses this electron to give a Na⁺ ion.
Chlorine has seven valence electrons. It gains one electron to give a Cl⁻ ion.
Both ions achieve a full outer shell of eight electrons, a stable configuration.
The +1 charge on Na⁺ and the –1 charge on Cl⁻ cancel, so the compound is overall neutral and the ions combine in a 1:1 ratio.
Chlorine has seven valence electrons. It gains one electron to give a Cl⁻ ion.
Both ions achieve a full outer shell of eight electrons, a stable configuration.
The +1 charge on Na⁺ and the –1 charge on Cl⁻ cancel, so the compound is overall neutral and the ions combine in a 1:1 ratio.
Examiner tips
- Show the electron transfer for each element; use the symbols Na⁺ and Cl⁻.
- Explain the octet rule as the reason for ion formation.
- State that the charges cancel to give a neutral compound.
- Use the 1:1 stoichiometry to justify the neutrality.
Common mistakes
- Writing that sodium *gains* an electron or chlorine *loses* an electron.
- Forgetting to mention the octet rule or the 1:1 ratio.
- Using the wrong charge symbols (e.g., Na⁻ or Cl⁺).
Mark scheme (5 marks)
- Sodium loses one electron to form a Na⁺ ion
- Sodium forms a positive ion because electrons have a negative charge / losing an electron leaves a positive charge
- Chlorine gains one electron to form a Cl⁻ ion
- Atoms form ions to achieve a full outer shell of 8 electrons / stable electron arrangement
- The +1 charge on Na⁺ and the −1 charge on Cl⁻ cancel / balance, giving an overall neutral compound with a 1:1 ratio of ions
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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