The ionic lattice

An ionic compound is a giant structure of ions. The ions have a regular, repeating arrangement called an ionic lattice.

An ionic lattice is held together by strong electrostatic forces of attraction between the oppositely charged ions. The forces act in all directions in the lattice. This is called ionic bonding.

Representing ionic compounds

Different types of model are used to represent giant ionic structures.

Sodium and chloride ions tightly packed in rows and columns.A two-dimensional space-filling model for the ionic lattice in sodium chlorideA three-dimensional space-filling model for the ionic lattice in sodium chlorideA three-dimensional space-filling model for the ionic lattice in sodium chlorideIonic structure of sodium chloride forming a cubic latticeA three-dimensional ball-and-stick model for the ionic lattice in sodium chloride

Remember that the lattice arrangement is giant - for example, a single grain of salt may contain 1.2 × 1018 (1,200,000,000,000,000,000) ions. The lattice arrangement continues in three dimensions. This is why solid ionic compounds form crystals with regular shapes.

Each has its advantages and limitations. For example:

  • the two-dimensional space-filling model clearly shows the arrangement of ions in one layer, but it does not show how the next layer of ions is arranged
  • the three-dimensional ball-and-stick model shows the arrangement of ions in a larger section of the crystal, but using sticks for bonds is misleading because the forces of attraction between ions actually act in all directions
  • the three-dimensional model is also misleading because it shows lots of free space between the ions, which there isn't
Question

Use the three-dimensional ball-and-stick model to show that the formula of sodium chloride is NaCl.

The model shows equal numbers of sodium ions and chloride ions, so the formula is NaCl.