Energy has to be transferred to a substance in order to melt or boil it. This energy is needed to overcome the forces between particles in the substance:
Ionic compounds are held together by electrostatic forces between the oppositely charged ions. These forces are usually referred to as ionic bonding. As the ionic lattice contains such a large number of ions, a lot of energy is needed to overcome this ionic bonding so ionic compounds have high melting and boiling points.
The strength of the ionic bonds depends on the charge on the ions. Ions with higher charge will have stronger forces between them, so will need more energy in order to overcome these forces.
|Compound||Melting point||Boiling point|
Ionic bonding between Mg2+ and O2- ions is stronger than that between Na+ and Cl- ions.
A substance can conduct electricity if:
Ionic compounds conduct electricity when melted to form a liquid or are 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.