How does the lattice structure of an ionic solid explain its melting point, brittleness, and conductivity?
Topic 2.3 Structure of Ionic Solids: describe the lattice of an ionic solid, relate lattice energy to ionic charge and size using Coulomb's law, and explain the properties of ionic compounds from their structure.
A focused answer to AP Chemistry Topic 2.3, covering the ionic lattice, lattice energy, the Coulombic dependence on charge and ionic radius, and how the lattice explains high melting points, brittleness and conductivity only when molten or dissolved, with worked reasoning.
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What this topic is asking
The College Board (Topic 2.3) wants you to describe the lattice of an ionic solid, to use lattice energy and Coulomb's law to compare ionic compounds, and to explain the characteristic properties of ionic solids (high melting points, hardness, brittleness, and conductivity only when molten or dissolved) directly from their structure.
The ionic lattice
There are no discrete molecules in an ionic solid; the formula (such as ) gives the simplest whole-number ratio of ions, not a molecule. The lattice geometry is the one that lets each ion sit as close as possible to oppositely charged neighbors while keeping like charges apart, which minimizes the total energy.
Lattice energy and Coulomb's law
This single relationship lets you rank ionic compounds. The charge product dominates: a compound of and ions (such as ) has a far greater lattice energy than one of and ions (such as ), because the charge product is four times larger. When charges are equal, the smaller ions win, because the shorter inter-ionic distance strengthens the attraction.
Properties from structure
Every characteristic property of ionic solids follows from the strong, extended lattice:
- High melting and boiling points. Melting requires overcoming many strong ionic attractions throughout the lattice, so it takes a lot of energy. Higher lattice energy means a higher melting point.
- Hard but brittle. The lattice resists deformation (hard), but a sharp blow can shift one layer so that ions of like charge become adjacent; they repel, and the crystal cleaves along that plane (brittle).
- Conductivity only when mobile. Conduction needs charge carriers that can move. In the rigid solid the ions are fixed, so it does not conduct; when molten or dissolved in water, the ions are free to move and the substance conducts.
This is the clearest demonstration in Unit 2 that structure dictates properties. The same Coulombic reasoning that set bond strength in Topic 2.2 now sets lattice energy, and lattice energy in turn predicts the melting point and hardness, while the geometry of the lattice predicts brittleness and the mobility of ions predicts conductivity. Being able to walk from "what are the charges and sizes of the ions" to "what will this substance be like" is exactly the reasoning chain the College Board rewards.
Try this
Q1. Explain why solid potassium chloride does not conduct electricity. [1 point]
- Cue. Its ions are locked in the lattice and cannot move, so there are no mobile charge carriers.
Q2. State two factors that increase the lattice energy of an ionic compound. [2 points]
- Cue. Larger ionic charges and smaller ionic radii (a shorter distance between ion centers).
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2023 (style)4 marksSection II (short FRQ). (a) Describe the arrangement of ions in a crystal of sodium chloride. (b) Predict whether or has the higher melting point and justify using lattice energy. (c) Explain why ionic solids are brittle. (d) Explain why conducts electricity when molten but not as a solid.Show worked answer →
A 4-point FRQ on ionic structure and properties.
(a) Arrangement (1 point): a regular three-dimensional lattice in which each is surrounded by ions and each by ions, alternating so that attractions are maximized and repulsions minimized.
(b) Melting point (1 point): is higher; its ions carry and charges versus and for , so by Coulomb's law the lattice energy is much greater and more energy is needed to melt it.
(c) Brittle (1 point): a blow shifts one layer so that like-charged ions align; the repulsion splits the crystal along that plane.
(d) Conductivity (1 point): conduction needs mobile charge carriers; in the solid the ions are locked in the lattice, but when molten they are free to move and carry current.
Markers reward a lattice description, a Coulombic lattice-energy comparison, the layer-shift explanation of brittleness, and mobile ions for conduction.
AP 2021 (style)1 marksSection I (multiple choice). Which ionic compound is expected to have the greatest lattice energy? (A) (B) (C) (D) . Justify your choice.Show worked answer →
A 1-point conceptual MCQ. The answer is (C).
Lattice energy increases with larger ionic charges and smaller ionic radii (Coulomb's law). has and ions, the largest charges of the four, while the others all involve and ions. The higher charge product dominates, giving the greatest lattice energy.
Related dot points
- Topic 2.2 Intramolecular Force and Potential Energy: interpret a potential-energy versus internuclear-distance curve to define bond length and bond energy, and explain how bond order, atomic size and charge affect bond strength.
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- Topic 2.1 Types of Chemical Bonds: classify bonds as ionic, covalent (polar or nonpolar), or metallic using electronegativity and the elements involved, and relate bond type to properties.
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- Topic 2.4 Structure of Metals and Alloys: use the electron-sea model to explain metallic properties, and describe how interstitial and substitutional alloys change those properties.
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- Topic 1.8 Valence Electrons and Ionic Compounds: relate the number of valence electrons to an element's group and reactivity, and predict the ions main-group elements form and the formulas of the ionic compounds they make.
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- Topic 2.7 VSEPR and Bond Hybridization: use VSEPR theory to predict molecular geometry and bond angles, assign the hybridization of the central atom, and relate geometry to molecular polarity.
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Sources & how we know this
- AP Chemistry Course and Exam Description — College Board (2020)