Sodium reacts with chlorine to form sodium chloride according to the balanced equation: 2Na + Cl₂ → 2NaCl. Explain, using the law of conservation of mass and your knowledge of ions, why the charges in sodium chloride balance and why the total mass of sodium chloride produced equals the total mass of sodium and chlorine that reacted.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sodium is a Group 1 metal and chlorine is a Group 7 non-metal. When they react, an ionic compound is formed.
Model answer (5 marks)
Sodium is in Group 1 and therefore has one valence electron. In the reaction it loses this electron to give a Na⁺ ion.
Chlorine is in Group 7 and has seven valence electrons. It gains one electron to give a Cl⁻ ion.
The Na⁺ ion has a +1 charge and the Cl⁻ ion has a –1 charge; the charges cancel so the compound NaCl is electrically neutral.
The law of conservation of mass states that no atoms are lost or made in a reaction. Thus the total mass of the 2 Na atoms and the 1 Cl₂ molecule equals the mass of the 2 NaCl units produced.
(1) Na → Na⁺ + e⁻
(2) Cl₂ + 2e⁻ → 2Cl⁻
(3) 2Na⁺ + 2Cl⁻ → 2NaCl
The ions combine in a 1:1 ratio, giving a neutral compound and the same total mass as the reactants.
Chlorine is in Group 7 and has seven valence electrons. It gains one electron to give a Cl⁻ ion.
The Na⁺ ion has a +1 charge and the Cl⁻ ion has a –1 charge; the charges cancel so the compound NaCl is electrically neutral.
The law of conservation of mass states that no atoms are lost or made in a reaction. Thus the total mass of the 2 Na atoms and the 1 Cl₂ molecule equals the mass of the 2 NaCl units produced.
(1) Na → Na⁺ + e⁻
(2) Cl₂ + 2e⁻ → 2Cl⁻
(3) 2Na⁺ + 2Cl⁻ → 2NaCl
The ions combine in a 1:1 ratio, giving a neutral compound and the same total mass as the reactants.
Examiner tips
- Show the electron transfer for each element to justify the ion charges. Explain that the charges cancel to give a neutral compound. State the conservation of mass and link it to the stoichiometry of the balanced equation. Use the correct ion symbols (Na⁺, Cl⁻) and mention the 8‑electron rule for stability.
Common mistakes
- Writing Na⁺ as Na⁻ or Cl⁻ as Cl⁺. Forgetting to mention that the charges cancel. Claiming atoms are lost or created instead of citing conservation of mass. Using the wrong ion charges for sodium or chlorine.
Mark scheme (5 marks)
- No atoms are lost or made during a chemical reaction (law of conservation of mass), so the mass of the products equals the mass of the reactants.
- Sodium is in Group 1 and loses one electron to form a Na⁺ ion (a positive ion).
- Chlorine is in Group 7 and gains one electron to form a Cl⁻ ion (a negative ion).
- The +1 charge on Na⁺ and the −1 charge on Cl⁻ cancel/balance, so the overall charge of sodium chloride is zero.
- Atoms form ions to achieve a full outer shell of 8 electrons, which is the most stable arrangement.
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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