Properties of ionic compounds

Ionic compounds have regular structures, called giant ionic lattices. In a giant ionic lattice, there are strong electrostatic forces of attraction acting in all directions between the oppositely charged ions. The structure and bonding of ionic compounds explain their properties.

High melting points and boiling points

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Ionic compounds have high melting and boiling points, so they are in the solid state at room temperature.

See the study guide on the three states of matter to see how bonding and structure are related to the properties of substances.

Energy must be transferred to a substance to make it melt or boil. This energy overcomes the strong electrostatic forces of attraction which act in all directions between the oppositely charged ions:

  • some forces are overcome during melting
  • all remaining forces are overcome during boiling

The more energy needed, the higher the melting point or boiling point. Since the electrostatic forces of attraction between oppositely charged ions are strong, their melting and boiling points are high.

Explanation

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.

CompoundMelting pointBoiling point
NaCl801°C1,413°C
MgO2,852°C3,600°C

Ionic bonds between Mg2+ and O2- ions are stronger than those between Na+ and Cl- ions

Conducting electricity

A substance can conduct electricity if:

  • it contains charged particles, such as ions, and
  • these particles are free to move from place to place

An ionic compound can conduct electricity when:

  • it has melted to form a liquid, or
  • it has dissolved in water to form an aqueous solution

Both these processes allow ions to move from place to place. Ionic compounds cannot conduct electricity in the solid state because their ions are held in fixed positions and cannot move.

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Ionic compounds conduct electricity when melted or in solution. They are insulators when solid.
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