Ionic Crystals (College Board AP® Chemistry)
Study Guide
Written by: Martín
Reviewed by: Stewart Hird
Ionic Crystals
Ionic crystals are arranged in a lattice structure of positive and negative ions
The lattice structure is the responsible of the crystalline nature of ionic compounds
The ions in the lattice are arranged in a three dimensional array with a regular repeating pattern
This systematic arrangement maximizes the Coulombic forces of attraction between positively charged ions and negatively charged ions
This arrangement also minimizes the Coulombic forces of repulsion between equally charged ions
Ionic lattices tend to be close-packed and symmetric arrangements
The overall charge of an ionic lattice is neutral
Ionic Lattice
Ionic crystals are arranged in lattice structures
Properties of Ionic Crystals
The physical properties of ionic crystals depend on the strength of the Coulombic attraction between the positive and negative ions
The strength of the Coulombic forces increase if:
The charge of the ions increase because the force of attraction is proportional to the magnitude of the charge
g. The coulombic force of attraction is stronger in MgCl2 than in NaCl
This occurs because Mg2+ has a 2+ charge and Na+ a 1+ charge
The ionic radius decrease because it is easier to attract an smaller ion rather than a bigger ion
g. The coulombic force of attraction is stronger in NaCl than in KCl
This occurs because Na+ has an smaller radius than K+
Table listing the characteristics of ionic crystals
Property | Explanation |
|---|---|
Brittle | Ions can be splitted apart |
High melting and boiling points | The Coulombic attraction between positive and negative ions in the lattice are strong |
Generally soluble in water | The lattice is broken down and the ions can form ion-dipole interactions with the water molecules |
Conduct electricity when in solution or molten | The lattice is broken down and there is free movement of ions |
Ionic Compound in a Solid and Molten / Solution
Molten or aqueous ions move freely but cannot in solid form
Examiner Tips and Tricks
It is important for your AP Chemistry exam that you know the physical properties of ionic crystals, and it is key to understand how to explain them using Coulomb’s Law and Periodic trends.
Worked Example
Based on Coulomb’s Law and using the information in the table below, which of the following anions is most likely to have the strongest attraction towards Na+ ions in a lattice structure?
Ion | Ionic Radius (pm) |
|---|---|
S2- | 185 |
Cl- | 182 |
Te2- | 147 |
Br- | 195 |
S2-
Cl-
Te2-
Br-
Answer:
The correct answer is A since S2- has the combination of most negative charge and the smallest radius, therefore the interactions with Na+ are going to be stronger
B and D are not correct since both of them are cations with a 1+ charge. Therefore, the Coulombic attractions are weaker
C is not correct. Even if Te2- has a 2- negative charge, its radius is bigger compared to S2-. Therefore, the forces of attractions are weaker in Te2-
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