AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/2/2-3)
Unit 2 · Topic 2.3
2.3 Structure of Ionic Solids
In an ionic solid, cations and anions pack into a repeating 3-D pattern called a lattice, arranged so that opposite charges are close and like charges are kept apart. Your particle drawings should show this alternating pattern, the correct ratio of ions and realistic relative sizes.
Key terms
- ionic solid
- crystal lattice
- cation
- anion
- ionic radius
- Coulombic attraction
What an ionic lattice is
An ionic solid isn't made of separate molecules. It's a continuous 3-D array in which every cation is surrounded by anions, and every anion by cations. This arrangement makes the attractions between opposite charges as large as possible and keeps repulsions between like charges as small as possible.
Because of this, the formula of an ionic compound, like NaCl or CaF₂, gives the ratio of ions, not a molecule. A single grain of table salt contains on the order of 10¹⁸ formula units, all locked into one lattice.
Drawing ionic solids
On the exam, you may need to draw or evaluate a particle diagram of an ionic solid, often as a 2-D slice. A good diagram follows these rules.
- Ions alternate: each cation's nearest neighbors are anions, and each anion's are cations.
- The ratio matches the formula. A slice of MgCl₂ has twice as many Cl⁻ as Mg²⁺.
- Relative sizes are sensible. Cations are smaller than their parent atoms and anions are larger, so in most common salts the anion is drawn larger. In NaCl, Cl⁻ (about 181 pm) is much larger than Na⁺ (about 102 pm).
- Ions are packed closely, touching their neighbors. There are no big empty gaps and no water unless the compound is dissolved.
Comparing ion sizes
Use the periodic trends from topic 1.7. Going down a group, ions of the same charge get larger: Li⁺ < Na⁺ < K⁺, and F⁻ < Cl⁻ < Br⁻. For ions with the same number of electrons, more protons means smaller: Na⁺ is smaller than F⁻ even though both have 10 electrons.
Size affects how strongly the ions attract (topic 2.2): smaller ions get closer together, so the attraction is stronger.
When you compare two diagrams, such as KCl and NaCl, the KCl drawing should show bigger cations and a larger distance between neighboring ion centers. That larger distance is the reason the attraction in KCl is weaker.
Why the lattice explains properties
The lattice model explains why ionic solids are hard, have high melting points and are brittle. If a force shifts one layer of ions by one position, like charges line up next to each other. The repulsion splits the crystal. Topic 3.2 uses this to explain properties of solids in more detail.
You won't be tested on specific crystal structures such as the names or geometry of unit cells. You only need the general alternating model.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Evaluating a student's drawing (classic trap)
A student draws a 2-D slice of solid LiF as a checkerboard of circles, with the Li⁺ circles drawn twice as large as the F⁻ circles. Identify the error and explain how to fix it.
Show the solutionHide the solution
- Step 1: The alternating checkerboard is fine: each Li⁺ is surrounded by F⁻ and vice versa, and there's a 1 : 1 ratio.
- Step 2: The sizes are wrong. Li⁺ has lost its only valence electron, leaving just 1s². F⁻ has 10 electrons in two shells.
- Step 3: Li⁺ (about 76 pm) is much smaller than F⁻ (about 133 pm), so the F⁻ circles should be drawn larger.
Answer: The relative sizes are reversed; F⁻ should be drawn larger than Li⁺ because Li⁺ has only one occupied shell while F⁻ has two.
- Example 2
Getting the ratio right
Describe a correct particle diagram for a small 2-D region of solid CaF₂.
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- Step 1: The formula tells you there are 2 F⁻ for every Ca²⁺.
- Step 2: Each Ca²⁺ should be surrounded by F⁻ ions, and F⁻ ions shouldn't be drawn touching other F⁻ ions as their closest neighbors with no Ca²⁺ nearby.
- Step 3: F⁻ is drawn somewhat larger than Ca²⁺, and the ions are closely packed.
Answer: Closely packed, with twice as many F⁻ as Ca²⁺, each Ca²⁺ surrounded by F⁻, and F⁻ drawn larger than Ca²⁺.
Common mistakes
- Drawing ionic compounds as separate molecules, such as Na–Cl pairs. An ionic solid is one continuous lattice.
- Drawing every cation larger than every anion. In most common salts the anion is larger; use periodic trends to compare.
- Getting the ion ratio wrong, for example a 1 : 1 ratio for MgCl₂.
- Leaving large gaps between ions. Ions in a solid are close-packed.
On the exam
- Expect to choose or draw a particle diagram that obeys Coulomb's law: alternating charges, correct ratio and sensible relative sizes.
- Explanations of brittleness or high melting points should mention the strong attractions between oppositely charged ions in the lattice.
Connected topics
Videos
Check yourself
4 questions on 2.3 Structure of Ionic Solids. Pick an answer to see if you got it, and why.
A student is drawing a two-dimensional particle model of one layer of solid LiCl. Which of the following descriptions gives the most accurate model?
When struck with a hammer, a crystal of KCl shatters, while a piece of potassium metal flattens. Which of the following best explains why the KCl crystal shatters?
Solid copper conducts electricity, but solid sodium chloride does not. Which of the following best explains the difference?
In solid NaF and solid CaO, the distances between the centers of neighboring cations and anions are nearly the same. Based on Coulomb's law, the attraction between neighboring ions in CaO is about how many times as strong as in NaF?
0 of 4 answered