Ionic compounds form when two atoms or groups of atoms share an ionic bond. Nearly all ionic compounds have a metallic cation (positively charged portion) and nonmetallic anion (negatively charged portion). The large electronegativity difference between the cation and anion makes the ionic bond extremely polar and results in certain characteristics. Here is a list of the properties of ionic compounds.
Summary of Properties of Ionic Compounds
- Crystalline solids
- Hard, brittle solids
- High melting and boiling points
- Don’t conduct electricity as solids, but do when molten or dissolved in water
- Often soluble in water and insoluble in nonpolar solvents
- High enthalpies of fusion and vaporization, compared with covalent compounds
- Solids have low vapor pressure
Explanation of Ionic Compound Physical Properties
Ionic compounds readily form crystals because the attraction between cations and anions is very strong and because the anions tend to be small in size relative to the size of a cation. It’s easy for the small anions to slip into the spaces between the large cations. The like charges repel each other, so the crystal arrangement keeps them separate. The result is a highly stable structure.
Hard and Brittle Solids
The stability of ionic crystals makes them hard, but they are brittle. A mechanical force, such as a strike with a hammer, shifts layers of ions. This brings ions with the same charge close to each other. The repulsion causes the crystal to shatter along a smooth plane of the crystal.
High Melting and Boiling Points
The high electronegativity difference strongly attracts the cation and anion. It takes a lot of energy (heat) to separate them to form liquids and gases.
Insulating Solids – Conducting Liquids
Ionic solids don’t conduct electricity very well. Rather, they are electrical insulators. However, melting the solid or dissolving it in water separates the electrical charges. Ionic compounds are electrical conductors in molten form or when dissolved in water.
Solubility in Polar and Nonpolar Solvents
By their very nature, ionic compounds are polar. They dissolve in polar solvents, like water, more readily than in nonpolar solvents, like oil or hexane.
High Enthalpies of Fusion and Vaporization
The enthalpy of fusion is the heat needed to melt one mole of solid at constant pressure. The enthalpy of vaporization is the heat needed to vaporize one mole of liquid under constant pressure. Ionic compounds have enthalpies of vaporization and fusion on the order of 10 to 100 times higher than that of most covalent compounds. Because of the high enthalpy values, ionic solids tend to have low vapor pressure. In other words, they often don’t have an odor. In humid air, some of an ionic compound may dissolve, revealing the scent.
A Familiar Example
It’s easier to remember the properties of ionic compounds if you consider the behavior of a familiar example: sodium chloride or table salt (NaCl). Salt forms crystals. You can see the cubic structure of salt crystal if you look closely or use a magnifying glass. Salt it hard, yet it’s easy to crush, so you know it’s brittle. A pile of salt is an electrical insulator, but if you dissolve the salt in water, it’s a strong electrolyte that conducts electricity. If you add salt to oil (a nonpolar solvent), it doesn’t dissolve. Salt also conducts electricity when you melt it, but it has a very high melting point of 800ºC. It’s easy to taste salt dissolved in water or saliva, but if you sniff dry salt you can’t smell it because it has a low vapor pressure.
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