AQA GCSE Chemistry 8462: Chemical bonds and ionic compounds – Revision Notes

Condensed recall for the final weeks. For full explanations and worked examples, use the chemical bonds and ionic compounds study guide.

These notes cover 4.2.1.1 Chemical bonds and 4.2.1.3 Ionic compounds from the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 onwards and exams in 2018 onwards (version 1.1). Both are in topic 2, assessed on Paper 1 at Foundation and Higher Tier. Neither subsection contains (HT only) content, so everything here is for both tiers. Electron transfer, dot and cross drawing and ion charges (4.2.1.2) are in the chemical bonds and ionic bonding revision notes. When you are ready, try the chemical bonds and ionic compounds practice questions. Course hub: AQA GCSE Chemistry. Every point is listed on the printable checklist.

4.2.1.1 The three strong bonds

Bond	Particles	Electrons are…	Occurs in
Ionic	Oppositely charged ions	transferred (metal to non-metal)	Compounds of metals with non-metals
Covalent	Atoms	shared in pairs	Most non-metallic elements; compounds of non-metals
Metallic	Atoms	delocalised and shared through the structure	Metallic elements and alloys

What attracts what (the electrostatic explanation):

• Ionic: positive ions ↔ negative ions.
• Covalent: the shared pair of electrons ↔ the nuclei of both atoms.
• Metallic: the delocalised electrons ↔ the positive metal centres.

All three are strong bonds, and all three are electrostatic attractions between opposite charges.

Model answer shape for “Explain the bonding in magnesium” (3 marks):

Magnesium is a metallic element, so the bonding is metallic.          [type + reason]
The outer-shell electrons are delocalised and shared through
the whole structure.                                                   [electrons]
The bond is the strong electrostatic attraction between the
delocalised electrons and the positive metal centres.                  [what attracts what]

Use the same three moves for ionic and covalent answers: type with a reason, what the electrons do, what attracts what.

Method: identify a bond type in steps

1. Write down the elements present.
2. Metal + non-metal (compound)        -> ionic
   Non-metal(s) only                   -> covalent
   Metal(s) only, element or alloy     -> metallic
   Single noble gas                    -> no strong bonds
3. Say how electrons are transferred or shared.
4. Say what is attracted to what.

4.2.1.3 Ionic compounds

Definition to learn word for word: an ionic compound is a giant structure of ions, held together by strong electrostatic forces of attraction between oppositely charged ions. The forces act in all directions in the lattice. This is ionic bonding.

Sodium chloride (the only ionic structure you need to know):

• Na⁺ and Cl⁻ ions alternate in a regular cubic lattice.
• Each ion has six oppositely charged ions as nearest neighbours.
• Cl⁻ ions are larger than Na⁺ ions.
• Equal numbers of each ion, so the formula is NaCl (ratio 1 : 1).
• No separate NaCl molecules exist.

Deducing “ionic” from a diagram

Clue in the diagram	Points to
Two types of particle, one + and one −	Ionic
Square brackets with charges in a dot and cross diagram	Ionic
Regular alternating pattern, drawn as part of a bigger structure	Giant lattice
Positive particles surrounded by “e⁻” with no negative ions	Metallic
Small separate groups of atoms joined by lines	Covalent (small molecules)

Limitations of each representation

Diagram	Main limitations for a giant ionic structure
Dot and cross	One pair of ions only; no 3D arrangement; no relative sizes; forces in all directions not shown
Ball and stick	Sticks look like bonds between particular pairs; ions shown too far apart; sizes may be wrong
2D	One layer only; ions above and below are missing
3D space-filling	Inner ions hidden; arrangement hard to see; forces not shown
All of them	Only a tiny fragment of a lattice that contains an enormous number of ions

A good limitation names a specific feature of that diagram and says what is wrong or missing.

Method: empirical formula from a model

1. Use the key to identify each ion.
2. Count each type of ion in the repeating pattern.
3. Divide by the highest common factor -> simplest whole-number ratio.
4. Write the formula: metal first, subscripts for numbers above 1.
5. Check: total positive charge = total negative charge.

Small worked reminders:

8 sodium ions, 4 sulfide ions       -> 8 : 4 = 2 : 1  -> Na2S   (2(+1) + (-2) = 0)
4 magnesium ions, 8 fluoride ions   -> 4 : 8 = 1 : 2  -> MgF2   ((+2) + 2(-1) = 0)

Written with subscripts these are Na₂S and MgF₂.

Edge warning: a small 3D block can give an uneven count. A 3 × 3 × 3 cube of a 1 : 1 lattice holds 14 of one ion and 13 of the other. A real crystal is so large that the edges do not matter, so the ratio is 1 : 1. Trust the repeating pattern and the charges, not a raw count from a tiny model.

Must-know distinctions

• Ionic vs metallic: both involve positive particles, but only ionic compounds contain negative ions. Metallic structures have delocalised electrons instead.
• Giant lattice vs molecule: an ionic formula gives a ratio of ions, not a molecule.
• Covalent element vs covalent compound: Cl₂ is a covalent element; H₂O is a covalent compound. Both are non-metals only.
• Alloy vs compound: an alloy of two metals is metallic, not ionic.
• Empirical formula vs ion count: Li₂O, not Li₁₂O₆.

Key terms

Ion – a charged particle formed when an atom loses or gains electrons. Lattice – a regular repeating 3D arrangement of particles. Giant structure – a lattice that continues in all directions with no separate molecules. Delocalised electrons – outer electrons free to move through a whole metallic structure. Empirical formula – the simplest whole-number ratio of the particles in a compound.

Quick self-test

1. Name the particles in a metallic bond.
2. What type of bonding is in solid potassium chloride?
3. What type of bonding is in liquid bromine?
4. Solder is an alloy of tin and lead. What type of bonding does it contain?
5. In a covalent bond, what is the shared pair of electrons attracted to?
6. Complete: “In an ionic lattice the electrostatic forces act in ______.”
7. How many Cl⁻ ions are nearest neighbours to one Na⁺ ion in sodium chloride?
8. A diagram shows a regular grid of “+” and “−” particles in equal numbers. What does this tell you about the compound, and what is its ratio of ions?
9. A model of an ionic compound has 10 potassium ions and 5 oxide ions. Give the empirical formula.
10. Give one limitation of a ball and stick model of sodium chloride.
11. Give one limitation of a 2D diagram of sodium chloride.
12. A diagram shows identical positive particles with electrons between them. Is it ionic? Explain.

Answers

1. Metal atoms sharing delocalised electrons (positive metal centres attracted to the delocalised electrons).
2. Ionic (potassium is a metal, chlorine a non-metal).
3. Covalent (bromine is a non-metallic element; atoms share a pair of electrons).
4. Metallic.
5. The nuclei of both atoms.
6. All directions.
7. Six.
8. It is ionic: two oppositely charged ions in a regular lattice. Ratio 1 : 1.
9. 10 : 5 = 2 : 1, so K₂O (check: 2(+1) + (−2) = 0).
10. Any one: sticks suggest bonds between particular pairs of ions, when forces act in all directions; ions are shown too far apart; sizes may not be to scale; only a small part of the lattice is shown.
11. Any one: shows only one layer, so ions above and below are missing; no 3D arrangement; only a small part of the lattice.
12. No. There are no negative ions; positive particles among delocalised electrons show metallic bonding.

Where marks are usually lost

• Calling an alloy such as brass or solder “ionic” because it contains two elements.
• Saying covalent bonding happens in “all” non-metallic elements. The noble gases do not easily form molecules.
• Describing a covalent bond as “sharing electrons” with no mention of a pair or of attraction to the nuclei.
• Writing “ionic bonds are between a sodium ion and a chloride ion”, which suggests pairs, instead of “forces act in all directions in the lattice”.
• Using “molecule” anywhere in an answer about sodium chloride.
• Giving “it’s not to scale” as a limitation without naming which feature is not to scale.
• Leaving an empirical formula unsimplified, or writing the non-metal first.
• Reading positive centres with electrons as an ionic diagram.
• Counting the ions in a tiny 3D block and writing an uneven formula without checking the charges.

Related resources

• Chemical bonds and ionic compounds study guide
• Chemical bonds and ionic compounds practice questions
• Chemical bonds and ionic bonding practice questions (electron transfer and dot and cross)
• All free 10-minute diagnostics

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016 onwards, exams in 2018 onwards, version 1.1 (4 October 2019), published by AQA – sections 4.2.1.1 Chemical bonds and 4.2.1.3 Ionic compounds.