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.

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.

CompoundMelting pointBoiling point
NaCl801°C1413°C
MgO2852°C3600°C

You can see from the data in the table that sodium chloride is a solid below 801°C, a liquid between 801°C and 1413°C, and a gas at temperatures higher than 1413°C.

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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