Magnesium oxide is an ionic compound formed when magnesium burns in oxygen. Explain how magnesium oxide is formed, including how the ions are produced and why the compound is held together.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Magnesium is a metal in Group 2 of the periodic table. Oxygen is a non-metal in Group 6. When magnesium burns in oxygen, a white solid, magnesium oxide, is formed.
Model answer (5 marks)
Magnesium is a Group 2 metal and has two valence electrons. When it burns in oxygen it loses these two electrons to give a Mg²⁺ ion.
Oxygen is a Group 6 non‑metal and requires two electrons to complete its valence shell. It gains the two electrons lost by magnesium to form an O²⁻ ion.
The Mg²⁺ and O²⁻ ions are then attracted to each other by strong electrostatic forces of attraction, forming an ionic bond. The resulting ionic lattice of Mg²⁺ and O²⁻ ions is the white solid magnesium oxide.
Oxygen is a Group 6 non‑metal and requires two electrons to complete its valence shell. It gains the two electrons lost by magnesium to form an O²⁻ ion.
The Mg²⁺ and O²⁻ ions are then attracted to each other by strong electrostatic forces of attraction, forming an ionic bond. The resulting ionic lattice of Mg²⁺ and O²⁻ ions is the white solid magnesium oxide.
Examiner tips
- Show the electron transfer step for each element; use the correct ion symbols (Mg²⁺, O²⁻).
- Explain that the attraction between opposite charges gives the ionic bond and holds the compound together.
Common mistakes
- Writing the reaction as a covalent bond or using a wrong charge for oxygen (O⁻).
- Forgetting to mention that magnesium loses electrons or that oxygen gains them.
Mark scheme (5 marks)
- Magnesium loses two electrons (to form a 2+ ion / Mg²⁺)
- Oxygen gains two electrons (to form a 2− ion / O²⁻)
- Both ions are formed / a metal atom loses electrons and a non-metal gains those electrons
- An ionic bond is formed between the oppositely charged ions (Mg²⁺ and O²⁻)
- The compound is held together by strong electrostatic forces (of attraction between the oppositely charged ions)
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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