KEY2CHEM

Ionic Compounds

Ionic compounds are composed of regular, repeating arrays of oppositely-charged particles in three dimensions. Opposite charges are attracted to one another, as described by Coulomb’s law; the force of attraction is directly proportional to the ion charges ($$q_1$$ and $$q_2$$) and inversely proportional to the square of the distance ( related to ionic radius, $$r$$) separating the ions.

$$F \propto \frac{q_1 \times q_2}{r^2}$$

The properties of ionic compounds can be described by their structure. For example, ionic compounds are brittle (cracks without deforming) because the application of external force causes like-charged ions to repel one another, and the crystal cracks (instead of denting or bending) if sufficient force is applied.

Example 1.

Which compound is expected to have the highest melting point?

A. $$\require{mhchem}\ce{MgO}$$

B. $$\require{mhchem}\ce{NaCl}$$

C. $$\require{mhchem}\ce{CsBr}$$

Solution

A. $$\require{mhchem}\ce{MgO}$$

The force holding ions together is described by Coulomb’s law: the force increases with increasing ion charge and with decreasing ion size. As such, the strongest interparticle forces (requiring the greatest energy to separate the particles, as with melting), will be found between ions with the largest magnitude of charge and the smallest sizes.

Example 2.

Why are ionic compounds generally brittle?

A. Their electrons are loosely held and fall off occasionally.

B. When like-charged ions align due to external force, they repel.

C. Application of external force decreases the interparticle attraction, and the material bends.

Solution

B. When like-charged ions align due to external force, they repel.

Strong electrostatic attractions hold the ions in an ionic compound in a specific position; application of external force moves them out of position, where they repel with like charges. This causes an ionic solid to crack instead of denting or bending.

Example 3.

Which is expected to be a characteristic of the following ionic compound?

A. hard

B. malleable

C. ductile

Solution

A. hard

Due to the regular repeating array of oppositely charged ions, ionic compounds are generally hard and brittle. Because the electrons are generally localized to each ion, ionic compounds are not malleable or ductile (unlike metals).