2.3 Structure of Ionic Solids

What Holds Ionic Solids Together

Ionic solids form when metals lose electrons to nonmetals, creating cations and anions held together by electrostatic (ionic) bonds. These attractions extend in all directions, producing a rigid, repeating crystal lattice structure that maximizes attractions between oppositely charged ions and minimizes repulsions between like-charged ions.

Lattice Energy Trends

Lattice energy is the energy released when gaseous ions form a solid lattice (or required to separate it). It increases with greater ionic charge and decreases with increasing ionic radius (distance between ion centers), following a relationship similar to Coulomb's law: energy is proportional to (Q1×Q2)/r. Higher lattice energy means a stronger, more stable lattice.

Properties from Structure

The strong, uniform electrostatic forces in ionic lattices explain their high melting/boiling points, brittleness, and poor electrical conductivity as solids (ions are fixed in place). However, when melted or dissolved in water, ions become mobile, allowing the substance to conduct electricity.

Key terms

Ionic bond

The electrostatic attraction between oppositely charged cations and anions.

Crystal lattice

The repeating three-dimensional arrangement of ions in an ionic solid that maximizes attraction and minimizes repulsion.

Lattice energy

The energy released when gaseous ions combine to form a solid ionic lattice; increases with higher charge and smaller ionic radius.

Why are ionic solids brittle?

Stress shifts ion layers so like-charged ions align and repel, cracking the lattice.

Conductivity of ionic solids

Poor as solids (ions fixed), but good when melted or dissolved (ions become mobile).

Try an AP-style question

Why are ionic solids brittle?

1. They have weak intermolecular forces between ions
2. Shifting layers brings like charges together, causing repulsion
3. They contain covalent bonds that break under stress
4. Their electrons are delocalized, so layers slip apart