You need to be able to recall and compare some of the properties of sodium chloride and magnesium oxide, both ionic compounds. The table summarises these properties.
|Sodium chloride, NaCI||Magnesium oxide, MgO|
|Melting point||High (801 °C)||Very high (2850 °C)|
|Can conduct electricity when solid?||No||No|
|Can conduct electricity when molten liquid?||Yes||Yes|
|Can conduct electricity when dissolved in water?||Yes||Yes (but not very soluble)|
Ionic bonds are the electrostatic forces of attraction between oppositely-charged ions.
The oppositely-charged ions are arranged in a regular way to form a giant ionic lattice. It is a 'lattice' because the arrangement is a regular one and 'giant' because the arrangement is repeated many times with large numbers of ions.
Ionic compounds often form crystals as a result. The diagram shows part of a sodium chloride ionic lattice.
The melting point of sodium chloride is lower than that of magnesium oxide because it has weaker ionic bonds, which need less heat energy to overcome. This is for two reasons:
For an ionic substance to conduct electricity, its ions must be free to move so that they can carry charge from place to place. Ions are free to move when an ionic compound is a molten liquid or in solution but not when it is solid.
Read on for more higher tier.
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