Magnesium oxide is an ionic compound formed when magnesium reacts with oxygen. Explain how magnesium oxide is formed from its elements, including why the ions formed have the charges they do, and why the overall compound has no overall charge.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Magnesium is in Group 2 of the periodic table. Oxygen is in Group 6.
Model answer (5 marks)
Magnesium atoms lose two valence electrons to give Mg²⁺ ions.
Oxygen atoms gain two electrons to give O²⁻ ions.
The loss and gain of electrons allow both ions to achieve a full outer shell, i.e. a noble‑gas configuration.
The charge of each ion is related to its group: Group 2 loses two electrons → 2+; Group 6 gains two electrons → 2−.
In the compound the 2+ charge of Mg²⁺ is exactly balanced by the 2− charge of O²⁻, so the overall charge is zero.
Oxygen atoms gain two electrons to give O²⁻ ions.
The loss and gain of electrons allow both ions to achieve a full outer shell, i.e. a noble‑gas configuration.
The charge of each ion is related to its group: Group 2 loses two electrons → 2+; Group 6 gains two electrons → 2−.
In the compound the 2+ charge of Mg²⁺ is exactly balanced by the 2− charge of O²⁻, so the overall charge is zero.
Examiner tips
- Use the command word ‘Explain’ to describe the process, not just state facts. Show the electron transfer and link it to the group numbers. Mention the 1:1 stoichiometry that gives charge balance.
Common mistakes
- Saying magnesium gives up one electron or oxygen gains one electron. Forgetting to explain why the ions are stable (noble‑gas configuration). Not stating that the charges cancel out in a 1:1 ratio.
Mark scheme (5 marks)
- Magnesium atom loses 2 electrons to form a Mg²⁺ ion
- Oxygen atom gains 2 electrons to form an O²⁻ ion
- Ions achieve a full outer electron shell / stable noble gas electron configuration
- The charges on the ions are linked to their group number — Group 2 element loses 2 electrons giving a 2+ charge; Group 6 element gains 2 electrons giving a 2− charge
- The compound has no overall charge because the 2+ charge on Mg²⁺ and the 2− charge on O²⁻ cancel out / balance each other in a 1:1 ratio
Key terms in this question
ionic compound · ion · overall charge
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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