Giant ionic structures

4 marks · Normal

Magnesium oxide (MgO) is a typical giant ionic compound.

(a)

State the ions present in magnesium oxide.

[1 mark]

(b)

Explain why magnesium oxide has a very high melting point.

[2 marks]

(c)

Describe what happens to the ions when solid magnesium oxide melts.

[1 mark]

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(a) Mg^{2+} and O^{2-} ions · must show Names or formulae of both ions

(b) Because strong electrostatic attractions between oppositely charged ions in the lattice require a large amount of energy to break. · must show strong electrostatic forces, many ions in lattice, a lot of energy needed

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Marking points

B1 (a) · 1 mark · States Mg^{2+} and O^{2-} ions (chemical symbols or names accepted).

M1 (b) · 1 mark · Identifies strong electrostatic attractions between oppositely charged ions in a giant lattice.

A1 (b) · 1 mark · States that a large amount of energy is required to overcome these forces.

B2 (c) · 1 mark · States ions become free to move (or can carry charge in molten state).

Worked solution

a) The ions present are Mg^{2+} and O^{2-}.

b) Magnesium oxide has a very high melting point because the giant ionic lattice contains strong electrostatic attractions between Mg^{2+} and O^{2-} ions. A large amount of energy is needed to overcome these forces.

c) When magnesium oxide melts, the ions are free to move and can carry an electric current.