Ionic bonding is easier to understand when you separate two ideas: electrons are transferred to form ions, then the electrostatic attraction between oppositely charged ions holds the ionic compound together.
What is ionic bonding?
An ionic bond is the strong electrostatic attraction between positive and negative ions. Ionic compounds usually form when a metal reacts with a non-metal. The metal loses one or more electrons and becomes a positive ion; the non-metal gains electrons and becomes a negative ion.
The three-stage idea
1. A metal loses electron(s). It becomes a positive ion.
2. A non-metal gains electron(s). It becomes a negative ion.
3. Opposite charges attract. That electrostatic attraction is the ionic bond.
Worked example: sodium chloride
Sodium has one electron in its outer shell that it can lose. Chlorine needs one electron to complete its outer shell.
Na → Na+ + e−
Cl + e− → Cl−
The resulting Na+ and Cl− ions attract one another. In solid sodium chloride, huge numbers of these ions are arranged in a giant ionic lattice.
Worked example: magnesium oxide
Magnesium loses two electrons to form Mg2+. Oxygen gains two electrons to form O2−. Their charges balance in a 1:1 ratio, giving the formula MgO.
Mg → Mg2+ + 2e−
O + 2e− → O2−
How to work out ionic formulae
The total positive charge and total negative charge in an ionic compound must balance. For example, Mg2+ needs two Cl− ions, so magnesium chloride is MgCl2. Aluminium ions are Al3+ and oxide ions are O2−, so the smallest whole-number ratio is Al2O3.
Why ionic compounds have high melting points
Ionic compounds contain a giant lattice of oppositely charged ions. Strong electrostatic attractions act in all directions through the lattice, so a large amount of energy is needed to overcome them.
When do ionic compounds conduct electricity?
Solid ionic compounds do not conduct because their ions are fixed in position. When molten, or when dissolved in water if the compound is soluble, the ions can move and carry charge, so the substance can conduct electricity.
Common ionic-bonding mistakes
Saying that electron transfer itself is the bond: electron transfer forms the ions; the bond is the attraction between opposite charges.
Giving ions the wrong charge: use periodic-table group information and electron arrangement to check how many electrons are lost or gained.
Forgetting the giant lattice: an ionic solid is not made from isolated pairs of ions. It contains a repeating three-dimensional lattice.
Saying solid sodium chloride conducts: charged particles are present, but the ions cannot move while the lattice is solid.
Quick practice
1. What ion does calcium form? 2. What ion does fluorine form? 3. Write the formula for calcium fluoride. 4. Explain why molten calcium fluoride conducts electricity.
Answers: 1. Ca2+. 2. F−. 3. CaF2. 4. The ions are free to move and can carry electrical charge.
Continue your GCSE Chemistry revision
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Frequently asked questions
What is the definition of an ionic bond?
An ionic bond is the strong electrostatic attraction between oppositely charged ions.
Why do metals form positive ions?
They lose electrons. Losing negatively charged electrons leaves the ion with an overall positive charge.
Why do ionic compounds conduct when molten but not solid?
In a solid lattice the ions are fixed. When molten, the ions can move and carry charge through the liquid.