Ionic bondingAQA A-Level Chemistry: Revision notes
Section 1
What is ionic bonding?
Ionic bonding is the electrostatic attraction between oppositely charged ions in a lattice. It is formed typically between a metal, which loses electrons to form a positive ion, and a non-metal, which gains them to form a negative ion.
An ionic compound is a giant lattice: a regular, repeating 3D arrangement of ions. Each ion is surrounded by oppositely charged ions, and the attraction acts in all directions. There are no separate molecules, so the formula, e.g. NaCl, shows only the simplest ratio of ions.
Saying ionic bonding is attraction between atoms, or that an ionic compound contains molecules. It is attraction between ions in a giant lattice.
Section 2
Predicting the charge on a simple ion
The charge on a simple ion comes from the group number in the Periodic Table, because atoms gain or lose electrons to reach a full outer shell:
| Group | 1 | 2 | 3 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|
| Ion charge | 1+ | 2+ | 3+ | 3− | 2− | 1− |
Examples: Na⁺, Mg²⁺, Al³⁺, N³⁻, O²⁻, Cl⁻.
Metals (left) form positive ions; non-metals (right) form negative ions.
Section 3
Compound ions
A compound ion is a group of atoms covalently bonded together with an overall charge. You must know these:
| Name | Formula |
|---|---|
| ammonium | NH₄⁺ |
| hydroxide | OH⁻ |
| nitrate | NO₃⁻ |
| carbonate | CO₃²⁻ |
| sulfate | SO₄²⁻ |
Ammonium is the only common positive compound ion in this list.
Section 4
Constructing formulas
The total positive charge must equal the total negative charge, so the compound is electrically neutral:
- Write the ions with their charges
- Find the smallest numbers of each ion that make the charges balance
- Write these as subscripts; use brackets around a compound ion if you need more than one
Examples:
- Mg²⁺ and Cl⁻: MgCl₂
- Al³⁺ and O²⁻: 2 × (3+) = 6+ and 3 × (2−) = 6−, so Al₂O₃
- Ca²⁺ and NO₃⁻: Ca(NO₃)₂
- NH₄⁺ and SO₄²⁻: (NH₄)₂SO₄
- Mg²⁺ and N³⁻: Mg₃N₂
Check by adding up the charges: the total must be zero. Never change the formula of a compound ion (e.g. NO₃⁻) to balance, only the number of ions.
Section 5
Describing ionic bonding in an answer
A full answer names the particles and the type of force:
- the compound is made of positive and negative ions (name them with their charges)
- there is electrostatic attraction between oppositely charged ions
- the ions are in a giant lattice, so the attraction acts in all directions
For MgO: Mg²⁺ and O²⁻ ions are held in a giant lattice by strong electrostatic attraction. The formula shows the 1 : 1 ratio because 2+ balances 2−.
Must Know
- Ionic bonding = electrostatic attraction between oppositely charged ions in a lattice
- Group 1, 2, 3 form 1+, 2+, 3+; Group 5, 6, 7 form 3−, 2−, 1−
- NH₄⁺, OH⁻, NO₃⁻, CO₃²⁻, SO₄²⁻
- Balance total charges to construct formulas; use brackets for compound ions
- Ionic compounds are giant lattices, not molecules
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Ionic bonding
- Magnesium oxide is used to line industrial furnaces because it has a very high melting point. Aluminium sulfate is used to treat drinking water.Describe the bonding in magnesium oxide.2 marks
- A garden fertiliser is a mixture of ammonium nitrate and ammonium sulfate. The manufacturer lists the ingredients using their formulas.Explain why the formula of ammonium sulfate contains two ammonium ions for every sulfate ion.2 marks
- A geologist records three minerals in a rock sample: bauxite, which contains aluminium oxide; limestone, which contains calcium carbonate; and a salt deposit containing calcium nitrate.Use the position of the elements in the Periodic Table to predict the charge on the ions in aluminium oxide, and construct its formula.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).