1.1 What is Chemistry?

http://www.ck12.org Chapter 8. Ionic and Metallic Bonding

Lattice Energy

There are a number of different ways to measure the strength of a given crystal lattice. One way would be to measure
the amount of energy needed to completely pull apart an ionic substance into isolated ions. This value, known as
thelattice energy, cannot be measured directly, but it can be calculated based on measured energy changes for
other more feasible processes. The lattice energy of an ionic solid provides us with one way to measure the relative
strength of the ionic bonds in that compound.Table8.1 shows the lattice energies for various ionic substances:

TABLE8.1: Lattice Energies for Some Ionic Compounds

Compound Lattice Energy (kJ/mol) Compound Lattice Energy (kJ/mol)

LiF 1030 KBr 671

LiCl 834 CsCl 657

LiI 730 CsI 600

NaF 910 MgCl 2 2326

NaCl 788 SrCl 2 2127

NaBr 732 MgO 3795

NaI 682 CaO 3414

KF 808 SrO 3217

KCl 701 ScN 7547

Properties of Ionic Compounds

Ionic compounds exhibit certain properties, some of which are listed below:

• All ionic compounds form crystals.


• Ionic compounds tend to have high melting points and boiling points.


• Ionic compounds are very hard and very brittle.


• Ionic compounds conduct electricity when dissolved in water.

The last property above requires some additional explanation. We are all familiar with the process of dissolution
on a large scale. If you stir a spoonful of salt into a glass of water, the salt crystals are broken down and seem to
disappear into the water. On the atomic level, thedissolutionof an ionic compound occurs when water interacts
with the ions in the crystal lattice, causing the lattice to break apart (Figure8.8):

Once the ions are dissolved, the presence of charged particles distributed throughout the liquid allows the solution
to conduct electricity (Figure8.9). The more ions that are freed from the lattice, the more conductive the solution
will be.

Below is a summary of some common ionic compounds and their practical applications (Table8.2):

TABLE8.2: Common Examples of Ionic compounds

Formula Name Common name Uses

NaCl Sodium chloride Table salt Food additive

NaHCO 3 Sodium hydrogen carbon-

ate

Baking soda Mild cleaner, antacid

NaOH Sodium hydroxide Lye Drain cleaner

NaF Sodium fluoride n/a Active ingredient in

toothpaste