When a metal element reacts with a non-metal element an ionic compound is formed. They have a giant lattice structure with strong ionic bonds. A lot of energy is needed to overcome these bonds.
The physical properties of ionic compounds can be explained by thinking about their structure and bonding.
Ionic compounds are solids at room temperature. Melting and boiling are state changes.
Energy has to be transferred to a substance in order to melt or boil it. This energy is needed to break the bonds between particles in the substance:
The more energy needed, the higher the melting point or boiling point.
Ionic compounds are held together by many strong electrostatic forces between the oppositely charged ions. These forces are usually referred to as ionic bonds. As the ionic lattice contains such a large number of ions, a lot of energy is needed to overcome these ionic bonds so ionic compounds have high melting and boiling points.
| Ionic compound | Melting point | Boiling point |
|---|---|---|
| NaCl | 801°C | 1,413°C |
| MgO | 2,852°C | 3,600°C |
Ionic bonds between Mg2+ and O2- ions are stronger than those between Na+ and Cl- ions.
A substance can conduct electricity if:
Ionic compounds conduct electricity when molten to form a liquid or dissolved in water to form an aqueous solution. This is because both processes make their ions free to move from place to place. Ionic compounds cannot conduct electricity when solid, as their ions are held in fixed positions and cannot move.