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Science

Ionic bonding

Giant ionic lattice

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, NaCIMagnesium oxide, MgO
Melting pointHigh (801 °C)Very high (2850 °C)
Can conduct electricity when solid?NoNo
Can conduct electricity when molten liquid?YesYes
Can conduct electricity when dissolved in water?YesYes (but not very soluble)

Structure and bonding

Ionic bonds are the electrostatic forces of attraction between oppositely-charged ions.

Ionic lattice of sodium chloride, showing positively charged sodium ions bonded to negatively charged chloride 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.

Higher tier

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:

  • The Na+ and Cl ions in sodium chloride have fewer charges than the Mg2+ and O2– ions in magnesium oxide
  • Na+ ions are larger than Mg2+ and cannot get as close to the negatively charged ions

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.

Back to The periodic table index

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