A grain of table salt is a tiny cube. Tap a salt crystal with a hammer and it shatters; heat it on a stove and nothing happens, because it only melts at 801 °C; put two wires in dry salt and no current flows. Each of these facts comes from how the ions in an ionic solid are arranged and how strongly they attract each other. This page builds that picture.
The arrangement of ions
Topic 2.1 described an ionic bond as the attraction between a cation and an anion. In a real sample there are not two ions but an enormous number of them. They settle into a repeating three-dimensional array, called a crystal lattice, in which every ion is surrounded by ions of the opposite charge. A solid built this way is an ionic solid.
Two points matter for the exam:
- No molecules. A Na⁺ ion in salt is not paired with one Cl⁻. It is attracted equally to all its Cl⁻ neighbors (six of them in salt), and each of those is attracted to six Na⁺. The formula NaCl gives only the ratio, 1 : 1.
- Ratio from charges. The lattice holds cations and anions in the ratio that makes the whole solid neutral: one Mg²⁺ for every two Cl⁻ in MgCl₂.
When you draw an ionic solid, alternate cations and anions, draw the anion larger when it is (Cl⁻ is larger than Na⁺), and keep the ratio of the formula. The exam does not ask you to name or recognize specific crystal arrangements; the alternating pattern is enough.
Lattice energy and Coulomb's law
The lattice energy is the energy needed to separate one mole of an ionic solid into ions far apart from each other. It measures how strongly the lattice is held together. By Coulomb's law (F ∝ q₁q₂/r²) it depends on two things:
- Charge. Larger ion charges attract more. A 2+/2− pair has a charge product of 4, four times that of a 1+/1− pair.
- Distance. Smaller ions sit closer together, and a shorter distance between ion centers gives a stronger attraction.
Charge usually wins: MgO (2+, 2−) has a far larger lattice energy than NaF (1+, 1−), even though the ions are at similar distances. When charges are equal, compare sizes.
Worked example. Which has the larger lattice energy, LiCl or KCl? Explain.
Claim: LiCl.
Evidence and reasoning: both contain a 1+ cation and Cl⁻, so the charges are the same. Li⁺ has electrons in only the first shell, while K⁺ has electrons in three shells, so Li⁺ is much smaller. The Li⁺–Cl⁻ distance is shorter, so by Coulomb's law the attraction between the ions is stronger and more energy is needed to separate them.
Notice the answer compares both compounds and names the cause (distance between ion centers), not just "Li is smaller."
Worked example. Estimate how many times stronger the attraction is between ions in MgO (centers 210 pm apart) than in NaCl (282 pm apart).
Ratio = (2 × 2 / 210²) ÷ (1 × 1 / 282²) = 4 × (282/210)² = 4 × 1.803 = 7.21.
About 7.21 times stronger. Most of that factor comes from the charges (4) and the rest from the shorter distance.
Properties explained
High melting temperatures. Melting means letting ions leave their fixed places and slide past each other. Every ion is held by several strong attractions in every direction, so a lot of energy is needed. Melting temperatures follow lattice energy: KCl 770 °C, NaCl 801 °C, NaF 993 °C, MgO 2852 °C.
Hardness and brittleness. Ionic solids are hard, because the strong attractions resist being pushed. But they are brittle: hit one and it shatters rather than bending. A sharp force shifts one layer of ions sideways by one ion's place. Now each cation sits beside cations and each anion beside anions. Like charges repel, the layers push apart, and the crystal splits along a flat face.
Electrical conductivity. A current is moving charge. In an ionic solid the charged particles are the ions, and they are locked in place, so the solid has very low electrical conductivity. Melt it, or dissolve it in water, and the ions can move: then it conducts well. The electrons stay on their ions throughout; it is the ions themselves that carry the charge.
| Condition | Ions | Conducts? |
|---|---|---|
| Solid | fixed in the lattice | no |
| Melted | move past each other | yes |
| Dissolved in water | separate and move independently | yes |
Common mistakes
- "Na–Cl molecules." Ionic compounds do not form molecules; draw a lattice, not pairs.
- Electrons carry the current in melted salt. The ions do.
- Size without charge. Compare charges first, then distances, and say what each does to the attraction for both compounds.
- Brittle means weak. Ionic solids are strong (high melting temperatures). They are brittle because a shifted layer brings like charges together.