AQA Chemistry 5.2.1.3 · Model
Ionic compounds
The formula says NaCl. So is a grain of salt made of little NaCl molecules, or of something else entirely?
Combined Science · Foundation tier
Start here
Crush a salt crystal. You get smaller cubes.
Look at table salt through a hand lens and every grain is a tiny cube with flat faces and square corners. Crush one and the fragments break along flat faces into smaller cubes. Grow salt slowly from a solution and you get cubes the size of dice.
Commit first
Why does salt come out as cubes at every size?
When sodium reacts with chlorine, the ions do not pair off. Each Na⁺ pulls on every Cl⁻ near it, and each Cl⁻ pulls on every Na⁺. They settle into a giant ionic lattice: a regular, three-dimensional arrangement in which positive and negative ions alternate in every direction. The strong electrostatic forces act in all directions at once. “Giant” means exactly that: a single grain holds around 10¹⁸ ions, and the pattern repeats the whole way through. The cube you see is the repeat you cannot.
The lattice lens
How many neighbours does one ion have?
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Commit first. In a real salt crystal, how many chloride ions sit right next to one sodium ion?
Think again
“The formula is NaCl, so salt is made of NaCl molecules: one sodium stuck to one chlorine.”
Spot the flaw
ExplainYou have just counted six neighbours. What does the formula NaCl really tell you?
Deduce the formula from the diagram
Count the ions. Write the simplest ratio.
The exam shows part of a lattice and asks for the empirical formula. Count each kind of ion in the diagram, then cancel down. Type the formula with plain numbers, for example Na2O.
No drawing of a lattice is the real thing. A dot-and-cross diagram shows where the electrons went but not the lattice. A 2D diagram shows the alternating pattern but only one layer. A ball-and-stick model shows the 3D repeat but draws the ions far apart and joins them with sticks, as if the force ran along a line. Every model trades one truth for another. AQA asks you to say which.
Sort the limitations
Which model has this limitation?
Each statement is a limitation of exactly one of the three models above.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- Give the features: regular, 3D, alternating, forces in all directions.
- Deduce
- Reach the formula from the diagram you are given. Show the count.
- Evaluate
- Give strengths and limitations, then judge.
Key fact
An ionic compound is a giant lattice of alternating positive and negative ions held by strong electrostatic forces in all directions. Its formula is the simplest ratio of ions, not a molecule.
Examiner tip
Key note · AQA 5.2.1.3
Ionic compounds
- Ionic compounds form a giant ionic lattice.
- The lattice is a regular 3D arrangement of alternating positive and negative ions.
- Strong electrostatic forces act between the ions in all directions.
- In NaCl each ion has six neighbours of the opposite charge.
- The formula is the simplest ratio of ions: deduce it by counting ions in a diagram.
- Every model misses something: dot-and-cross shows no lattice, 2D shows one layer, ball-and-stick shows sticks and gaps.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask Mr Badmus AI
Stuck counting ions in a lattice diagram?
Lattice figures are two-dimensional slices or small cut-outs of a crystal that extends in three dimensions. In sodium chloride the chloride ion is larger than the sodium ion; the figures draw them at roughly the right relative size. The ball-and-stick model is drawn with gaps and sticks on purpose, so that its limitations can be discussed.