Fig. 5.1 The diagram shows a model of the giant structure of solid sodium chloride. Each sphere represents an ion, and the + and − signs show the charge on ...

This question is about the giant ionic lattice of sodium chloride and how its structure explains melting point and conductivity. A useful way to picture the arrangement is a repeating three-dimensional grid in which the charges alternate, so no two like charges sit next to each other.

(a) [1] The structure is a giant ionic lattice (giant ionic structure) [1].

(b) [2] The ions are arranged in a regular, repeating pattern [1]; the charges alternate, so each positive ion (Na⁺) is surrounded by negative ions and each negative ion (Cl^-) is surrounded by positive ions [1], as shown above.

(c) [1] The ions are held together by electrostatic attraction (ionic bonding) between the oppositely charged ions [1].

(d) [2] Sodium chloride has a high melting point because there are strong electrostatic forces of attraction between the oppositely charged ions throughout the lattice [1]; a large amount of energy is needed to break down the lattice (overcome these forces) [1].

(e) [2] Solid sodium chloride does not conduct because its ions are held in fixed positions and cannot move [1]; there are therefore no free charged particles to carry the current [1]. (The electrons are all tightly bound within ions, so they cannot move either.)

(f) [2] When dissolved in water, sodium chloride does conduct [1]; the solid dissolves and the ions separate and become free to move, so they can carry the charge through the solution [1].

Exam tip: conduction always comes back to one idea, whether solid, molten or dissolved: are there charged particles that are free to move? For ionic substances only melting or dissolving frees the ions.