High Melting and Boiling Points: Significant thermal energy is required to overcome the strong electrostatic attractions throughout the giant lattice. Consequently, most ionic compounds are solids at room temperature with very high melting points.
Electrical Conductivity: In the solid state, ions are held in fixed positions within the lattice and cannot move to carry a charge. However, when molten or aqueous, the lattice breaks down, allowing the ions to move freely and act as mobile charge carriers.
Brittleness: Ionic crystals are brittle because a sharp blow can cause layers of ions to shift. This shift brings ions of the same charge into alignment, resulting in strong electrostatic repulsion that causes the crystal to shatter along cleavage planes.
Ionic Charge: As the magnitude of the charge on the ions increases, the electrostatic attraction becomes significantly stronger. For example, a ion will attract a ion much more strongly than a ion attracts a ion, leading to higher melting points.
Ionic Radius: Smaller ions can get closer to one another in the lattice, which increases the strength of the electrostatic attraction according to Coulomb's Law. Therefore, compounds containing smaller ions generally have higher lattice enthalpies and melting points.
Charge Density: The combination of high charge and small radius results in high charge density. Ions with high charge density exert a more powerful pull on neighboring ions, creating a more robust and heat-resistant lattice structure.
| Property | Ionic Compounds | Covalent Molecular |
|---|---|---|
| Bonding Type | Electrostatic attraction between ions | Shared pairs of electrons |
| Structure | Giant 3D Lattice | Discrete Molecules |
| Conductivity | Only when liquid/aqueous | Generally non-conductive |
| Melting Point | High (breaking strong bonds) | Low (breaking weak intermolecular forces) |
Identify the Lattice: When asked to explain properties, always start by identifying the structure as a giant ionic lattice. Failing to mention the 'giant' nature or the 'lattice' often results in lost marks.
Use Precise Terminology: Always refer to 'ions' and 'electrostatic attraction' rather than 'atoms' or 'intermolecular forces'. Ionic compounds do not have intermolecular forces because they are not made of molecules.
Explain Conductivity Correctly: Ensure you specify that ions are the mobile charge carriers in molten ionic liquids. A common mistake is to claim that 'electrons' move, which is only true for metallic bonding.
Compare Trends: If asked to compare two ionic compounds, look at the charges first, then the radii. Charge usually has a more significant impact on the melting point than radius does.
