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.
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
Assessment in practice
2 marks in each selected HL structured example marksHow it is assessed
Questions describe ionic bonding in a lattice or compare conductivity of a solid ionic compound with a metal.
Command terms
describe / explain
What earns marks
Refer to electrostatic attraction or the lattice, then explain ion fixation in a solid or ion mobility in a molten/dissolved state.
Watch for
Saying only that a compound is ionic without linking the requested property to lattice structure and particle mobility.
Representative question
Question 1
[Maximum number: 2]
Predict, with a reason, the electrical conductivity of K(s) and KCl(s).
K(s):
KCl(s) :
K( s) :
high «electrical conductivity»/conductor AND electrons free to flow/delocalized
KCl(s):
low «electrical conductivity»/not a conductor AND ions/charged particles are fixed in position
Marking guidance:
Award [1 max] for K(s) is a conductor AND KCl(s) is not a conductor.
Do not accept just "metal" or "metallic bonding" for M1.
Do not accept an explanation in terms of electrons for M2.
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?