Structure of Ionic Solids
Represent an extended ionic solid with the correct ratio, charge balance and mobile-charge explanation.
AP Chemistry Topic 2.3 · CED printed p. 47 / PDF p. 54. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Ionic neutrality
- Attraction and repulsion
Think repeated array, not isolated pairs
An ionic solid contains cations and anions throughout a systematic three-dimensional array. A formula such as NaCl gives the overall 1:1 ratio; it does not mean that each Na+ belongs to one separate NaCl molecule. Each ion interacts with many neighbors. A plausible model places nearby unlike charges so attraction stabilizes the array, while avoiding unnecessary close like-charge neighbors. The arrangement balances all attractive and repulsive terms; repulsions do not magically disappear.
A drawing on paper is often a two-dimensional slice. Label that limitation. Show the ion identities, charges, relative sizes when known, and the required overall ratio. If the formula is MgCl2, a model with equal numbers of Mg2+ and Cl− fails neutrality. You do not need to memorize coordination numbers, named unit cells or exact crystal structures for this AP objective.
Use the array to explain behavior
Strong electrostatic attractions make many ionic solids difficult to melt. In the solid, ions are charged but held near fixed sites, so they cannot provide bulk charge transport. When melted, mobile cations and anions can carry current. Some dissolve to form mobile aqueous ions, but solubility is a separate competition of interactions and is not guaranteed for all salts.
If a layer shifts, ions of like charge can become adjacent. The resulting repulsion favors fracture, explaining brittleness. A model showing only isolated attractive cation–anion pairs cannot explain that layer-slip behavior well. The useful AP response connects an observed property to the repeated-particle arrangement and its mobility, rather than saying “it is ionic, so it has ionic properties.”
Let's work through it
Work it through
Evaluate a proposed solid model for MgCl2 with four Mg2+ circles and four Cl− circles.
- Count charges: four Mg ions contribute +8 charge units; four chloride ions contribute −4. The depicted sample is not neutral.
- Keep four Mg ions and use eight Cl ions to satisfy Mg:Cl = 1:2 and total charge zero.
- Arrange a repeated three-dimensional model with favorable unlike-charge contacts; a flat drawing can be labeled as a slice or projection.
- Do not connect each Mg to only two Cl and call each trio a molecule. Formula units state ratio in an extended ionic solid.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Why does a charged-ion solid usually fail to conduct?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Why does a charged-ion solid usually fail to conduct?
Worked answer and reason
Its ions are held near lattice sites, so it lacks mobile charge carriers for bulk current.
Charge presence and charge mobility are different conditions.
Retrieval 2
What charge ratio must a particle model of Al2O3 show?
Worked answer and reason
Two Al3+ for every three O2−, giving equal positive and negative total charge.
A structural model must honor stoichiometry and neutrality.
Catch the tempting mistake
- NaCl consists of separate NaCl molecules.
- The formula represents an ion ratio in an extended lattice.
- Ionic solids are brittle because the ions have weak attractions.
- The attraction is strong; shifting layers can create repulsive like-charge contacts.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 2 problem 5 · ap application · particle
- Unit 2 problem 6 · ap synthesis · multiple
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For the teacher
- Ask for ratio, charge total and a clearly stated slice/projection limit.
- Students should explain conductivity through mobility, not through charge alone.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 2.3.A
- Represent an ionic solid with a particulate model that is consistent with Coulomb’s law and the properties of the constituent ions.
- 2.3.A.1
- The cations and anions in an ionic crystal are arranged in a systematic, periodic 3-D array that maximizes the attractive forces among cations and anions while minimizing the repulsive forces.
- Knowledge of specific crystal structures is not essential to an understanding of the learning objective and will not be assessed on the AP Exam.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
Glossary and deeper reading · 5 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Cation
A cation is a positively charged ion. Its total positive nuclear charge exceeds its total electron charge. For example, calcium commonly forms a cation with charge +2 by losing two electrons.
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Anion
An anion is a negatively charged ion. It has more electrons than needed to balance its positive nuclear charge. For example, a chlorine atom gaining one electron forms a chloride anion.
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Ion
An ion is an atom or group of atoms with a net electric charge. Ordinary ion formation changes electron count without changing the element's proton count. For example, a sodium atom losing one electron becomes a sodium ion with charge +1.
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Aqueous solution
An aqueous solution is a solution in which water is the solvent. The dissolved substance may originate as a solid, liquid, or gas, so aqueous does not mean that a solid has melted. For example, sodium chloride dissolved in water is aqueous even though the pure salt is solid.
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Solubility
Solubility describes how much of a substance can dissolve in a specified solvent under stated conditions. It depends on the solute, solvent, temperature, and other relevant conditions and is distinct from how fast dissolution occurs. For example, two salts can have different equilibrium solubilities in water at the same temperature.