Melting, boiling and electrical conductivity of ionic compounds

An ionic compound contains positive and negative ions arranged in a regular structure called an ionic lattice. Strong electrostatic attractions between oppositely charged ions hold the lattice together.

Melting and boiling points

To melt or boil an ionic compound, energy must be supplied to overcome the attractions between its ions. These attractions are strong, so a large amount of energy is needed. Ionic compounds therefore generally have high melting and boiling points.

Electrical conductivity

Electrical conduction requires charged particles that can move and carry charge through a substance. Ionic compounds contain charged ions, but whether they conduct depends on whether those ions are free to move.

• Solid: The ions are fixed in position in the lattice. They cannot carry charge through the solid, so it conducts poorly.
• Molten: The lattice has broken down and the ions can move. They carry charge through the liquid, so it conducts electricity.
• Dissolved in water: The ions separate and can move through the solution. They carry charge through the solution, so it conducts electricity.

Giant ionic lattice structure

An ionic compound consists of positive and negative ions arranged in a regular pattern that repeats in three dimensions. This extended arrangement is called a giant ionic lattice.

Each ion is surrounded by ions of the opposite charge. Because opposite charges attract, strong electrostatic forces of attraction act between the ions throughout the lattice, in all directions. These attractions hold the structure together.

An ionic crystal is not made of separate small molecules. The whole crystal is one extended lattice of ions, held together by attractions between oppositely charged ions.