Not a pair but a lattice

It is tempting to picture sodium chloride as one Na⁺ paired off with one Cl⁻. That picture is wrong, and the reason matters.

A charge does not point in a direction. The positive charge on a sodium ion attracts every chloride ion near it, not one chosen partner. So the attraction is never confined to a pair: each ion pulls on all its neighbours at once.

Six neighbours each

In solid sodium chloride the ions settle into the arrangement that lets this work out evenly. Every Na⁺ ion is surrounded by six Cl⁻ ions, and every Cl⁻ ion is surrounded by six Na⁺ ions, repeating in every direction.

Figure to be added

The sodium chloride crystal lattice, drawn both as an open cube showing the six neighbours of an ion and as packed spheres (figure 10.7).

Ionic lattice

The regular three-dimensional network into which large numbers of positive and negative ions arrange themselves, held by the attractions between them. The ions of every ionic compound are arranged as a lattice of this kind.

This is why a grain of salt has flat faces and square corners. The shape you can see is the arrangement of the ions inside it, scaled up.

Activity 01

Build the lattice

Using coloured clay balls, plastic beads or any other suitable material, build a model of the sodium chloride ionic lattice. Use one colour for the sodium ions and another for the chloride ions, and check as you build that each ion of one colour ends up with six neighbours of the other.

No such thing as an NaCl molecule

The formula NaCl gives the ratio of ions in the lattice — one sodium ion for every chloride ion — not a count of atoms in a molecule. A crystal of salt is one continuous lattice, not a heap of NaCl pairs.