An ionic bond is formed when there is a large electronegativity difference between the elements participating in the bond. The greater the difference, the stronger the attraction between the positive ion (cation) and negative ion (anion).
Properties Shared by Ionic Compounds
The properties of ionic compounds relate to how strongly the positive and negative ions attract each other in an ionic bond. Iconic compounds also exhibit the following properties:
- They form crystals.
Ionic compounds form crystal lattices rather than amorphous solids. Although molecular compounds form crystals, they frequently take other forms plus molecular crystals typically are softer than ionic crystals. At an atomic level, an ionic crystal is a regular structure, with the cation and anion alternating with each other and forming a three-dimensional structure based largely on the smaller ion evenly filling in the gaps between the larger ion.
- They have high melting points and high boiling points.
High temperatures are required to overcome the attraction between the positive and negative ions in ionic compounds. Therefore, a lot of energy is required to melt ionic compounds or cause them to boil.
- They have higher enthalpies of fusion and vaporization than molecular compounds.
Just as ionic compounds have high melting and boiling points, they usually have enthalpies of fusion and vaporization that can be 10 to 100 times higher than those of most molecular compounds. The enthalpy of fusion is the heat required melt a single mole of a solid under constant pressure. The enthalpy of vaporization is the heat required for vaporize one mole of a liquid compound under constant pressure.
- They're hard and brittle.
Ionic crystals are hard because the positive and negative ions are strongly attracted to each other and difficult to separate, however, when pressure is applied to an ionic crystal then ions of like charge may be forced closer to each other. The electrostatic repulsion can be enough to split the crystal, which is why ionic solids also are brittle.
- They conduct electricity when they are dissolved in water.
When ionic compounds are dissolved in water the dissociated ions are free to conduct electric charge through the solution. Molten ionic compounds (molten salts) also conduct electricity.
- They're good insulators.
Although they conduct in molten form or in aqueous solution, ionic solids do not conduct electricity very well because the ions are bound so tightly to each other.
A Common Household Example
A familiar example of an ionic compound is table salt or sodium chloride. Salt has a high melting point of 800ºC. While a salt crystal is an electric insulator, saline solutions (salt dissolved in water) readily conduct electricity. Molten salt is also a conductor. If you examine salt crystals with a magnifying glass, you can observe the regular cubic structure resulting from the crystal lattice. Salt crystals are hard, yet brittle -- it's easy to crush a crystal. Although dissolved salt has a recognizable flavor, you don't smell solid salt because it has a low vapor pressure.