Properties of ionic compounds

AQA·Chemistry (Combined Science)·5.2.2.3·Trilogy — Higher

Giant ionic lattices and ionic bonding

An ionic compound is made of positive and negative ions arranged in a regular, repeating pattern. This pattern extends in three dimensions to form a giant ionic lattice. The compound is not made of separate molecules: the ions form one continuous structure.

Positive and negative ions have opposite charges, so they attract each other. Each ion attracts oppositely charged ions around it in all directions. These strong electrostatic forces of attraction extend throughout the lattice and hold it together. This attraction between oppositely charged ions is called ionic bonding.

Sodium chloride is a typical example. Its lattice contains sodium ions, $$\ce{Na+}$$, and chloride ions, $$\ce{Cl-}$$, arranged in a regular repeating pattern. Each sodium ion is attracted to chloride ions around it, and each chloride ion is attracted to sodium ions around it.

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.