2.12 - Ionic Compounds

The properties of an ionic compound

• An ionic compound is built from huge numbers of ions packed together into a giant structure.
• Ionic compounds share a characteristic set of properties.

High melting and boiling points

• Ionic compounds have high melting points and high boiling points.
• Sodium chloride (table salt) melts at 801 °C and boils at 1,413 °C.

Soluble in water

Most ionic compounds dissolve in water.

Conducting electricity

• As a solid, an ionic compound can't conduct electricity.
• When molten or dissolved in water, it conducts electricity.

The giant ionic lattice - Why it explains those properties

• Oppositely charged ions are attracted to each other by strong electrostatic forces acting in all directions.
• This forms a giant ionic lattice: a regular, repeating 3D arrangement of alternating positive and negative ions.
• In sodium chloride, each sodium ion (Na⁺) is surrounded by six chloride ions (Cl^-), and each chloride ion is surrounded by six sodium ions.

Why melting and boiling points are high

• A giant lattice is held together by many strong ionic bonds.
• Breaking these strong electrostatic attractions throughout the lattice requires a large amount of energy, so ionic compounds have high melting and boiling points.

Why conductivity depends on state

• In a solid, ions are fixed in position in the lattice and can't move, so they can't carry an electric charge.
• When molten or dissolved in water, the lattice breaks apart and the ions are free to move and carry charge.

Why the formula is a ratio

The lattice is a continuous structure with no separate molecules, so its formula is the simplest whole-number ratio of positive to negative ions that balances the overall charge to zero (e.g. NaCl, MgCl₂).