Write a neutral formula
An ionic compound has zero total charge. Use the smallest whole-number ratio of ions that balances the positive and negative charges.
Lesson 2.3 · AQA Combined Science
Ionic formulae, lattices and properties
Three examples
| Ions | Charge balance | Formula |
|---|---|---|
| Na⁺ and Cl⁻ | +1 − 1 = 0 | NaCl |
| Mg²⁺ and O²⁻ | +2 − 2 = 0 | MgO |
| Mg²⁺ and Cl⁻ | +2 − 1 − 1 = 0 | MgCl₂ |
| Na⁺ and O²⁻ | +1 + 1 − 2 = 0 | Na₂O |
The small ₂ in MgCl₂ means two chloride ions for each magnesium ion. MgO is the simplest ratio, not Mg₂O₂.
Ions form a giant lattice
Sodium chloride is a giant ionic lattice, with a regular repeating arrangement of ions. Strong attractions act in all directions. There are no separate NaCl molecules. The picture is a small 2D section of a 3D lattice.
Why a high melting point?
A lot of energy is needed to overcome the strong electrostatic attractions between oppositely charged ions. Ionic compounds have high melting and boiling points.
When does an ionic compound conduct?
Solid: ions are fixed in position, so there are no mobile charged particles to carry current. Molten or dissolved in water: ions can move and carry charge. It is the ions, not free electrons, that conduct.
Remember
Ionic formulae balance charges. Ionic compounds conduct when molten or dissolved because their ions can move.
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