2.3 Ionic Solids

Structure of Ionic Solids

Rotate a 3D array of ions drawn at real ionic radii for 9 compounds. Compare charge and size through Coulomb's law, then shove one layer sideways until like charges meet and the crystal cleaves.

Particulate ModelCoulomb's LawCharge vs SizeBrittle Fracture
Topic 2.3

Structure of Ionic Solids

Represent an ionic solid with a particulate model that is consistent with Coulomb’s law and the properties of the constituent ions.

Cations and anions in an ionic crystal are arranged in a systematic, periodic three-dimensional array that maximizes attractions between oppositely charged ions while minimizing repulsions between like charges. That single sentence explains the whole topic.

The consequence at the particulate level is alternation: each cation is surrounded by anions and each anion by cations, in every direction. Draw it that way — a checkerboard, not clumps of like ions.

Three macroscopic properties follow directly:

  • High melting and boiling points, low vapor pressure. Melting requires overcoming a whole network of strong Coulombic attractions, not just a few.
  • Brittleness. Strike the crystal and one layer slides. Suddenly like charges face each other, the repulsion is enormous, and the crystal shatters along the plane. This is why ionic solids are brittle rather than malleable — a favorite AP explanation question.
  • Conductivity only when ions are mobile. Solid ionic compounds do not conduct: the ions are locked in place. Melt them or dissolve them and they conduct well.

Relative size also shapes the drawing. Cations are smaller than their parent atoms and anions are larger, so in a particulate diagram of NaCl the Cl⁻ ions should be drawn noticeably bigger than the Na⁺ ions.

Key points

  • The lattice arrangement maximizes attractions and minimizes repulsions — say this explicitly in free-response answers.
  • Brittleness comes from layer displacement bringing like charges into contact.
  • Conductivity requires mobile charge carriers: molten or aqueous, never solid.
  • Draw anions larger than cations in any particulate model of an ionic solid.

Common mistakes

  • Ionic solids do not conduct as solids. "Ionic compounds conduct electricity" is only true molten or dissolved.
  • Do not draw ion pairs. There is no NaCl "molecule" — the formula gives a ratio within an extended lattice.
  • Do not memorize crystal structures. The CED explicitly excludes specific structures such as face-centered cubic.
  • Size matters in your drawing. Drawing Na⁺ larger than Cl⁻ can cost a point on a particulate-representation question.

Worked example

Explain, at the particulate level, why solid MgCl₂ does not conduct electricity but molten MgCl₂ does.

In the solid, Mg²⁺ and Cl⁻ ions are held in fixed positions in the crystal lattice by strong Coulombic attractions. Electrical conduction requires charged particles that can move; the ions cannot translate, and the electrons are localized on the ions rather than delocalized, so there are no mobile charge carriers.

When the solid melts, enough energy has been supplied to overcome the lattice attractions. The ions are now free to move throughout the liquid. Applying a potential drives Mg²⁺ toward the negative electrode and Cl⁻ toward the positive electrode, so charge flows and the molten salt conducts.

Full notes for topic 2.3 →