The key idea
Ionic bonds form when electrons are transferred from a metal to a non-metal.The resulting oppositely charged ions attract each other electrostatically.Ionic compounds form giant lattice structures with high melting points.
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Electron transfer and ion formation
Ionic bonding occurs when a metal atom transfers one or more electrons to a non-metal atom.The metal loses electrons and becomes a positive cation; the non-metal gains electrons and becomes a negative anion.Both ions achieve a full outer electron shell, which is a stable arrangement.The electrostatic attraction between the oppositely charged ions is the ionic bond.
Giant ionic lattice
Ionic compounds do not exist as individual pairs of ions; instead, millions of ions pack together in a regular, repeating three-dimensional arrangement called a giant ionic lattice.Sodium chloride forms a cubic lattice in which each Na⁺ ion is surrounded by six Cl⁻ ions and vice versa.The strong electrostatic forces throughout the lattice give ionic compounds high melting and boiling points.
Properties of ionic compounds
Ionic compounds are generally solid at room temperature due to their high melting points.They are brittle because shifting the layers of ions brings like charges alongside each other, causing repulsion.They do not conduct electricity when solid (ions in fixed positions) but do conduct when molten or dissolved in water because the ions are then free to move and carry charge.
Writing ionic formulae
To write the formula of an ionic compound, balance the total positive and negative charges so the compound is neutral.For example, Al³⁺ and O²⁻: to balance 3+ and 2−, you need 2 Al³⁺ (total 6+) and 3 O²⁻ (total 6−), giving Al₂O₃.The formula must always be the simplest whole-number ratio of ions.
Definitions to learn
Ionic bond
The electrostatic attraction between oppositely charged ions formed by electron transfer.
Giant ionic lattice
A regular three-dimensional arrangement of billions of oppositely charged ions held together by electrostatic forces.
Cation
A positively charged ion formed by a metal atom losing electrons.
Anion
A negatively charged ion formed by a non-metal atom gaining electrons.
Electrostatic attraction
The force of attraction between positive and negative charges.
Lattice energy
The energy required to separate all ions in one mole of an ionic compound (higher = stronger bonding).
Describe the formation of magnesium chloride (MgCl₂) from its elements.
Magnesium has 2 outer electrons; chlorine has 7.
Each magnesium atom transfers one electron to each of two chlorine atoms.
Mg forms Mg²⁺; each Cl forms Cl⁻.
The ratio is 1 Mg²⁺ to 2 Cl⁻, giving MgCl₂.
Mg²⁺ and 2Cl⁻ ions are held together by electrostatic attraction in MgCl₂.
When predicting ionic formulae, always balance the total positive charge against the total negative charge.Show the working by listing the ion charges.
Do not say ions share electrons. Ionic bonding involves electron transfer, not sharing.
How to score full marks
- 1Always state that ionic bonding involves electron TRANSFER — never 'sharing' (that is covalent bonding).
- 2For conductivity, always link to whether ions CAN MOVE — fixed in solid (no), free in molten or solution (yes).
- 3When writing formulae, show the balance: 2 × Al³⁺ = 6+, 3 × O²⁻ = 6−, so Al₂O₃.
Try these yourself
Open each answer only after you have explained the full chemical process.
1State what type of bonding is present in sodium chloride.[1 mark]
- 1.Identify metal and non-metal.
2Describe how a sodium ion is formed from a sodium atom.[2 marks]
- 1.Consider what happens to the outer electron.
3Explain why ionic compounds have high melting points.[3 marks]
- 1.Link the lattice structure to the energy needed.
4Predict the formula of calcium oxide from the charges of the ions (Ca²⁺ and O²⁻).[2 marks]
- 1.Balance the charges so the compound is neutral.
5Explain why solid sodium chloride does not conduct electricity.[3 marks]
- 1.Consider whether ions can move.
6Explain why molten sodium chloride conducts electricity but solid sodium chloride does not.[3 marks]
- 1.Compare ion movement in each state.
7Write the formula of aluminium oxide given that aluminium forms Al³⁺ ions and oxygen forms O²⁻ ions.[2 marks]
- 1.Balance total positive and negative charges.
8Explain, in terms of structure and bonding, why ionic compounds dissolve in water and conduct electricity when dissolved.[3 marks]
- 1.Describe ion separation and mobility.