Ionic bonding – IGCSE Chemistry exam questions with mark scheme

1Grade 1–33 marks

Magnesium reacts with oxygen to form magnesium oxide, MgO.

Describe, in terms of electrons, how magnesium oxide is formed from magnesium atoms and oxygen atoms. Give the charges on the ions formed. [3]

magnesium atom loses / transfers two electrons(1)
to an oxygen atom / oxygen atom gains two electrons(1)
ions formed are Mg²⁺ and O²⁻(1)
Answer: Mg transfers 2 electrons to O, forming Mg²⁺ and O²⁻
Examiner tip: Say 'transfers' or 'loses/gains' — saying the atoms 'share' electrons describes covalent bonding and scores zero.

2Grade 4–5Triple only5 marks

Sodium chloride has a giant ionic lattice structure.

(a) Explain why sodium chloride has a high melting point. [3]

(b) Explain why sodium chloride conducts electricity when molten but not when solid. [2]

(a) strong electrostatic attraction / strong forces of attraction (between ions) (allow strong ionic bonds; reject intermolecular forces / covalent bonds / molecules: scores 0 for (a))(1)
(a) between oppositely charged ions / between Na⁺ and Cl⁻ ions (allow between positive and negative ions)(1)
(a) a lot of (heat) energy is needed to overcome these forces / attractions (throughout the giant lattice)(1)
(b) molten: ions are free to move (and carry charge)(1)
(b) solid: ions are held in fixed positions / cannot move (reject any reference to electrons moving – max 1)(1)
Answer: (a) strong electrostatic attraction between oppositely charged ions in a giant lattice; much energy needed to overcome it (b) molten: ions free to move; solid: ions held in fixed positions
Examiner tip: Any mention of 'intermolecular forces' or 'molecules' in an ionic compound loses marks — there are no molecules in NaCl.

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