Structure and Properties of Ionic Compounds
Quick Notes
- Ionic compounds form giant 3D lattice structures made of alternating positive and negative ions.
- We represent ionic compounds by their empirical formula (e.g. NaCl, MgO).
- Key physical properties:
- Low volatility (high melting/boiling points) due to strong electrostatic attractions.
- Electrical conductivity:
- No conductivity when solid (ions fixed in place).
- Conduct when molten or dissolved (ions free to move).
- Solubility:
- Often soluble in polar solvents (e.g. water).
- Usually insoluble in non-polar solvents.
- Lattice enthalpy measures ionic bond strength. Higher lattice enthalpy = stronger bonds = higher melting points.
- Lattice enthalpy increases with greater ionic charge and smaller ionic radius.
Full Notes
Lattice Structure of Ionic Compounds
Ionic compounds form giant ionic lattices:
- A regular, repeating 3D arrangement of oppositely charged ions.
- Strong electrostatic forces (ionic bonds) hold the structure together in all directions.
Example: Sodium chloride (NaCl) is made up of Na+ and Cl− ions attracted to each other in a giant repeating structure.
Sometimes lattice structures are shown as unit cells with ions on the corners of cubes. Remember in reality, ions are actually touching each other and the structure extends in 3D.
Ionic compounds are represented by empirical formulas (smallest whole-number ratio of ions).
- NaCl: each Na+ is surrounded by 6 Cl− ions, and vice versa.
- MgO: similar lattice but with stronger attraction between Mg2+ and O2−.
Physical Properties Explained
Most ionic compounds share similar physical properties, which can be explained by ionic bonding.
Volatility
Volatility is how easily a substance vaporises.
Ionic compounds have very low volatility (very high melting/boiling points) due to strong ionic bonds that require a lot of energy to break.
Electrical Conductivity
In solid state ionic compounds do not conduct electricity because ions are fixed in the lattice.
In molten or aqueous form they do conduct because ions are free to move and carry charge.
Solubility
Ionic compounds are often soluble in water (a polar solvent) (water molecules surround and stabilise ions)
Usually insoluble in non-polar solvents (e.g. hexane), which can't stabilise the charged ions
Lattice Enthalpy and Bond Strength
Lattice enthalpy is the energy released when 1 mole of an ionic solid forms from gaseous ions.
It reflects the strength of the ionic bond.
A more negative lattice enthalpy = stronger ionic attraction = higher melting point.
Factors affecting lattice enthalpy:
- Ion charge: greater charge = stronger attraction = more negative lattice enthalpy.
- Ion radius: smaller ions = stronger attraction (more negative lattice enthalpy).
Summary
- Ionic compounds form 3D lattice structures with alternating positive and negative ions.
- They have low volatility and high melting/boiling points.
- They conduct electricity when molten or in aqueous solution.
- They are soluble in polar solvents like water but not in non-polar solvents.
- Lattice enthalpy depends on ionic charge and radius.