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2.1.3—Ionic compounds

Syllabus
First assessment 2025
Objective
2.1.3
Level
HL

Ionic Lattices: Structure, Strength and Conductivity

An ionic lattice is a three-dimensional, repeating arrangement of cations and anions. Its empirical formula gives the simplest whole-number ion ratio, not a molecule.

the crystal is linked to a magnified three-dimensional lattice fragment; smaller Na+ and larger Cl- ions alternate in the lattice; the ions are not presented as discrete NaCl molecules or covalent pairs.
the 4 by 4 array alternates eight cations and eight anions; every horizontal and vertical nearest neighbour has opposite charge; two-headed arrows show attraction in all lattice directions; no like-charge attraction or covalent bond is shown.

Lattice dissociation enthalpy is the positive enthalpy change for separating one mole of a solid lattice into gaseous ions. It becomes larger when ionic charges are higher or ionic radii are smaller, because the electrostatic attraction is stronger.

Property Structure-based explanation
High melting point / low volatility Strong electrostatic attractions act throughout the lattice
Brittle A layer shift can bring like charges together, causing repulsion and fracture
Solid conductivity Ions are fixed and cannot carry charge through the solid
Molten/aqueous conductivity Ions are mobile and can carry charge
Solubility Depends on the balance between lattice attraction and ion–solvent attraction

Use charge density to compare lattice strength: MgO has stronger attractions than NaCl because both ions carry ±2 rather than ±1, so its melting point is higher. For conductivity, the presence of charged particles is not enough—solid NaCl does not conduct until its ions can move. Water often hydrates ions, but solubility still depends on the energy balance rather than on polarity alone.

Explaining Ionic-Compound Properties

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Predict, with a reason, the electrical conductivity of K(s) and KCl(s).

K(s):
KCl(s) :

The Ionic Model Summary

Retrieve the chain: atoms gain or lose electrons to form ions; oppositely charged ions attract and balance into empirical formulae; the three-dimensional lattice explains volatility, solubility, and conductivity.

When checking an answer, ask: Did I state gain or loss and charge? Did I define the bond as electrostatic attraction? Did I connect the property to lattice arrangement and ion mobility?