Properties of Ionic Solids (College Board AP Chemistry)

• 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 / forces that hold the particles of solids together
• Based on these bonds / forces, we can classify solids into:
  • 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 Solid Crystal Lattice

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

• The strength of an ionic bond increases as the magnitude of the charges increase and as the ions get smaller

Properties of Ionic Solids

• Ionic solids generally have high melting points and high boiling points
  • This is a result of the strong electrostatic attraction between the oppositely charged ions
• Despite the high melting and boiling points of ionic solids, they are poor conductors of electricity in solid state
  • This is because the valence electrons are confined to the negative ions and are not free to move
  • However, ionic solids can conduct electricity in solution and molten state due to the presence of mobile ions
• 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 similar charged ions and the breakdown of the crystal lattice

Brittleness of Ionic Solids

The image shows the effect of applying a shear stress to an ionic solid. The crystal separates along a plane of atoms and leads to repulsive interaction between ions with like charges

• Examples of ionic solids are:
  • Cesium chloride
  • Sodium chloride (Common salt)
  • Zinc sulfide (also known as zinc blende)