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C3.4 Giant Ionic Structures

GCSE Combined Science · AQA · Chapter C3: Structure and Bonding

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: the structure of ionic compounds and how it explains their melting points and electrical conductivity. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the structure of an ionic compound as a giant lattice of oppositely charged ions
  • Work out the formula of an ionic compound from a diagram or model of its structure
  • Explain why ionic compounds have high melting and boiling points
  • Explain why ionic compounds conduct electricity when melted or dissolved in water, but not when solid
  • Describe the limitations of different models of ionic structures

Key words

  • Giant ionic lattice: A huge 3D arrangement of oppositely charged ions held together by ionic bonds in all directions.
  • Lattice: A regular, repeating arrangement of particles.
  • Molten: Melted: turned into a liquid by heating.
  • Electrical conductor: A substance that lets an electric current flow through it. It needs charged particles that can move.

The structure of an ionic compound

In an ionic compound, millions of ions are packed together in a regular pattern called a giant ionic lattice. Each ion is surrounded by ions of the opposite charge. The strong electrostatic forces act in all directions, so every ion is held tightly in place.

Because the ions are arranged in a regular, repeating pattern, ionic compounds form crystals with flat faces and straight edges.

Key fact: A giant lattice keeps going in all directions. The formula, e.g. NaCl, shows only the ratio of ions, not the number in one crystal.

Working out a formula from a model

Count each type of ion in a repeating part of the diagram, then find the simplest whole-number ratio. Check that the charges add up to zero.

Worked example: formula from a model

A layer of a model has 10 metal ions (1+) and 5 non-metal ions.
Ratio = 10 : 5 = 2 : 1, so two metal ions are balanced by one non-metal ion.
Each non-metal ion must have a charge of 2−, since 2 × (+1) + (−2) = 0.

Answer: formula M₂X, e.g. sodium oxide, Na₂O

High melting and boiling points

To melt an ionic compound, the ions must be able to move past each other. That means overcoming the strong electrostatic forces between very many ions. This needs a lot of energy, so ionic compounds have high melting and boiling points. They are all solids at room temperature.

  • sodium chloride, NaCl; Ions Na⁺, Cl−; Melting point (°C) 801
  • potassium bromide, KBr; Ions K⁺, Br−; Melting point (°C) 734
  • magnesium oxide, MgO; Ions Mg²⁺, O²−; Melting point (°C) 2852

Magnesium oxide has a much higher melting point than sodium chloride. Its ions have larger charges (2+ and 2−), so the attraction between them is stronger and more energy is needed to overcome it.

Exam tip: Say “lots of energy is needed to overcome the strong electrostatic forces of attraction between oppositely charged ions”. Never say “intermolecular forces” for an ionic compound.

Conducting electricity

A current is a flow of charged particles. Ionic compounds contain charged ions, but they can only carry a current if the ions can move.

  • solid; Conducts no; Why ions are held in fixed positions in the lattice
  • molten (melted); Conducts yes; Why the lattice breaks down; ions are free to move and carry charge
  • dissolved in water (aqueous); Conducts yes; Why the ions separate and are free to move through the solution

In a circuit, the moving ions are attracted to the electrode with the opposite charge: positive ions (e.g. Na⁺, Cu²⁺) move to the negative electrode, and negative ions (e.g. Cl−, O²−) move to the positive electrode. This is how charge flows through the liquid.

Many ionic compounds dissolve in water, but not all of them do (for example, calcium carbonate and silver chloride are insoluble). An insoluble ionic compound cannot conduct as a solution, but it still conducts when molten. Sea water conducts much better than pure water because it contains lots of dissolved ions.

Models of ionic structures and their limitations

  • dot-and-cross; Good at showing how the ions form by electron transfer; Limitation no lattice, no 3D shape, no ion sizes
  • ball-and-stick; Good at showing the 3D arrangement and the ratio of ions; Limitation ions are not really far apart; sticks are not real bonds; shows only a small part
  • space-filling (close-packed); Good at showing how the ions pack together and their relative sizes; Limitation hard to see ions inside the structure

Common mistakes

  • Saying ionic compounds conduct because electrons move. In ionic compounds the ions move and carry the charge.
  • Saying solid sodium chloride has no ions. The ions are there; they just cannot move.
  • Talking about “molecules” of sodium chloride. Ionic compounds are giant structures, not molecules.
  • Saying “ionic bonds are hard to break” without saying there are many strong bonds acting in all directions.

Revision questions

Try these, then check your answers below each one.

Question 1. Potassium chloride is tested in three forms, A, B and C. In which beakers will the bulb light? (Choose from: B and C, A only, A and B, A, B and C.)

Answer: B and C

Working: In the solid (A) the ions are held in fixed positions. When molten (B) or dissolved (C) the ions can move and carry charge, so the bulb lights.

Question 2. Why do ionic compounds have high melting points? (Choose from: Many strong electrostatic forces between oppositely charged ions must be overcome, The covalent bonds inside the molecules are strong, The intermolecular forces are strong, The delocalised electrons hold the ions together.)

Answer: Many strong electrostatic forces between oppositely charged ions must be overcome

Working: A giant ionic lattice has very many strong ionic bonds acting in all directions. A lot of energy is needed to overcome them.

Question 3. True or false? Solid sodium chloride conducts electricity.

Answer: False

Working: False. The ions are fixed in the lattice. It conducts only when molten or dissolved, when the ions are free to move.

Related lessons

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This lesson is section C3.4 (AGCSC2004) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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