Revision notes for AQA GCSE Chemistry Ionic compounds. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Ionic compounds
What you'll learn
- What an ionic compound is and why it is called a giant structure.
- How ionic bonding holds ions together in a lattice.
- How sodium chloride can be represented using different models.
- How to work out an empirical formula from a model or diagram.
Before you start: ions and charges
An atom is normally neutral because it has the same number of negative electrons as positive protons. If an atom loses or gains electrons, it becomes charged.
Definition
Ion
An ion is a charged particle formed when an atom, or group of atoms, loses or gains electrons. A positive ion has lost electrons. A negative ion has gained electrons.
In many GCSE examples, metals form positive ions and non-metals form negative ions. For example, sodium forms Na+ ions and chlorine forms Cl− ions.
Opposite charges attract
Positive and negative charges attract each other. Two positive charges repel, and two negative charges repel.
Definition
Electrostatic force
An electrostatic force is a force of attraction or repulsion between charged particles.
This matters because ionic compounds are built from charged ions, not neutral atoms.
Ionic compounds are giant lattices
An ionic compound is not made from small separate molecules. Instead, it has a huge repeating arrangement of ions.
Definition
Ionic compound
An ionic compound is a giant structure of ions held together by strong electrostatic forces of attraction between oppositely charged ions.
Definition
Lattice
A lattice is a regular, repeating three-dimensional arrangement of particles.
Sodium chloride, NaCl, is the main ionic structure you need to be familiar with. It contains Na+ ions and Cl− ions arranged alternately in a giant ionic lattice.
Key Idea
The big idea
In an ionic compound, each ion is attracted to oppositely charged ions around it. These strong electrostatic attractions act in all directions through the lattice.
Ionic bonding
Definition
Ionic bonding
Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions in a giant ionic lattice.
A really important detail: ionic bonding is not just one attraction between one positive ion and one negative ion. The forces act throughout the whole lattice.
That is why diagrams showing just one Na+ next to one Cl− are only a very simplified way of thinking about sodium chloride.
Common Mistake
Calling sodium chloride a molecule
Sodium chloride is not made of NaCl molecules. The formula NaCl tells you the simplest ratio of ions: one Na+ ion for every one Cl− ion in the lattice.
Recognising an ionic compound from a diagram
A diagram is likely to show an ionic compound if it shows:
- positive and negative ions
- a regular repeating pattern
- a giant structure rather than separate small groups
- oppositely charged ions next to each other throughout the structure
Example
Recognising a giant ionic lattice
A diagram shows particles labelled X+ and Y− arranged in a repeating grid. The particles alternate between positive and negative, and the pattern continues in all directions.
- The labels X+ and Y− show that the particles are ions, because they have charges.
- The alternating pattern places oppositely charged ions next to each other, so electrostatic attractions can form.
- The pattern repeats across the diagram rather than forming separate pairs, so it represents a giant lattice.
- Therefore, the diagram shows an ionic compound with ionic bonding.
Sodium chloride: what NaCl means
The formula NaCl means that sodium chloride contains Na+ ions and Cl− ions in a 1:1 ratio.
It does not mean there is one sodium atom covalently joined to one chlorine atom. In a crystal of sodium chloride, there are huge numbers of ions packed together in a regular arrangement.
Tip
Formula meaning
For ionic compounds, the formula usually gives the simplest whole-number ratio of ions in the lattice, not the number of atoms in one molecule.
Working out empirical formulae from models
Definition
Empirical formula
The empirical formula is the simplest whole-number ratio of atoms or ions of each element in a compound.
For ionic compounds, the empirical formula is found by comparing how many of each ion are present in the model.
The method
- Count how many of each type of ion are shown.
- Write the numbers as a ratio.
- Simplify the ratio to the smallest whole numbers.
- Write the formula using subscripts where needed.
- Check that the total positive charge and total negative charge balance.
Example
Finding an empirical formula from an ion model
A model contains 4 Mg2+ ions and 8 Cl− ions. Work out the empirical formula.
- Count the ions: there are 4 magnesium ions and 8 chloride ions.
- Write the ratio Mg:Cl as 4:8.
- Simplify the ratio by dividing both numbers by 4, giving 1:2.
- Write the formula as MgCl2. The subscript 1 after Mg is not written.
- Check the charges: one Mg2+ ion has a 2+ charge, and two Cl− ions have a total 2− charge, so the formula is neutral overall.
Tip
Charge balance check
A correct ionic formula should have no overall charge. The total positive charge must balance the total negative charge.
Different models of ionic compounds
Scientists use models because real ionic lattices are far too small to see directly. Each model is useful, but each one also has limitations.
Dot-and-cross diagrams
A dot-and-cross diagram uses dots and crosses to show electrons from different atoms.
For ionic bonding, dot-and-cross diagrams can show that electrons have been transferred and that ions have full outer shells.
However, they do not show the full giant lattice. They often make the compound look like one positive ion and one negative ion, which is not the complete structure.
Ball-and-stick diagrams
A ball-and-stick diagram represents ions as spheres connected by lines.
This is useful because it can show the regular three-dimensional pattern of the lattice. You can see that positive and negative ions alternate.
The limitation is that the “sticks” are not real. Ionic compounds do not contain physical rods between ions. The lines represent the arrangement and attractions, not covalent bonds.
Space-filling diagrams
A space-filling diagram shows ions as spheres packed closely together.
This gives a better idea of how ions fill space in a crystal. It can also suggest the relative sizes of ions.
The limitation is that it can be harder to see the pattern inside the lattice, because the front ions may hide the ones behind.
Two-dimensional diagrams of three-dimensional structures
A two-dimensional diagram is a flat drawing of something that is really three-dimensional.
These diagrams are helpful in exams because they are simpler to draw and interpret. But they can make the lattice look like a flat sheet, even though the real structure extends in all directions.
Example
Describing a model limitation
A ball-and-stick model of sodium chloride shows Na+ and Cl− ions joined by lines. Give one useful feature and one limitation of this model.
- Identify the model type: spheres joined by lines means it is a ball-and-stick model.
- State what it shows well: it shows the regular repeating arrangement of Na+ and Cl− ions in the lattice.
- State a limitation: the lines may suggest that there are physical sticks or covalent bonds between ions, but ionic bonding is actually electrostatic attraction acting in all directions.
The key wording examiners like
For this topic, accurate wording matters. A strong answer about ionic bonding should include:
- strong electrostatic forces of attraction
- between oppositely charged ions
- acting in all directions
- in a giant ionic lattice
You do not need to know the detailed structures of ionic compounds other than sodium chloride, but you do need to understand how to interpret diagrams and models of giant ionic structures.
Exam technique
In the exam
- When identifying an ionic compound from a diagram, mention both ions with opposite charges and a regular giant lattice.
- When describing ionic bonding, avoid saying “shared electrons”; instead write “strong electrostatic forces of attraction between oppositely charged ions”.
- When finding a formula from a model, count the ions, simplify the ratio, write the positive ion first, and check that the charges balance.
Self review
Check yourself
- What does the formula NaCl tell you about the ratio of ions in sodium chloride?
- Why is it incorrect to describe sodium chloride as being made of molecules?
- A model contains 3 X2+ ions and 6 Y− ions. What is the empirical formula?
Chemical bonds, ionic, covalent and metallic
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