Properties of Ionic Solids (College Board AP® Chemistry): Revision Note

Properties of Ionic Solids

• A solid consists of structural units that are attracted to one another strongly enough to give a rigid substance

  • These structural units can be atoms, molecules, or ions

• The physical properties and structure of solids are dictated by the type of bonds or forces that hold the particles of solids together

• Based on these bonds and forces, solids fall into four types:

  • Molecular solids

  • Metallic solids

  • Ionic solids

  • Covalent network solids

Structure of ionic solids

• An ionic solid is made up of cations (positive ions) and anions (negative ions) held together by an electrostatic attraction between the oppositely charged ions

  • This electrostatic attraction is commonly called an ionic bond

• The ions in an ionic solid are arranged in a regular pattern of alternating positive and negative ions to give a solid crystal lattice

Ionic crystal lattice

A diagram showing the regular arrangement of alternating positive and negative ions that make up an ionic solid

• The attraction between the ions is stronger when the ion charges are larger and when the distance between the ion centers is smaller (smaller ions can get closer together)

Physical properties of ionic solids

• Ionic solids generally have low vapor pressures, high melting points and high boiling points

  • This is a result of the strong electrostatic attraction between the oppositely charged ions

• Ionic solids are poor conductors of electricity in the solid state

  • This is because the ions are held in fixed positions in the lattice, so they cannot move to carry charge

  • However, an ionic compound conducts electricity when it is melted (in a molten state) or dissolved in water or another solvent, because its ions are then free to move

• Unlike metallic solids, ionic solids are brittle and break apart when stress is applied to the solid

  • The applied stress causes the regular cation-anion arrangement to change into cation-cation and anion-anion arrangement

  • This causes repulsion between the similarly charged ions, and the crystal breaks apart

Brittleness of ionic solids

When a shear stress makes one layer of ions slide across another, ions with like charges line up, repel each other, and the crystal splits

• Examples of ionic solids are:

  • Cesium chloride

  • Sodium chloride (common salt)

  • Zinc sulfide (also known as zinc blende)