Properties of Ionic Compounds

Physical Properties

Ionic compounds have distinctive physical properties due to their structure and bonding:

• High melting and boiling points: Ionic compounds consist of many ions held together by strong electrostatic forces in a giant ionic lattice. These forces require a lot of energy to overcome, so ionic compounds melt and boil at very high temperatures.
• Solid at room temperature: Because of the strong ionic bonds, ionic compounds form solids at room temperature with a regular, repeating pattern of ions.
• Brittle and hard: Ionic compounds are hard because of the strong forces holding ions in place. However, they are brittle because when a force is applied, ions of the same charge can be forced next to each other, causing repulsion and the lattice to shatter.

For example, sodium chloride (table salt) has a high melting point of about 8016C and is solid at room temperature. It is hard but will break if struck sharply.

Electrical Conductivity

Electrical conductivity is the ability of a substance to allow electric current to pass through it, which depends on the movement of charged particles.

The ability of ionic compounds to conduct electricity depends on whether the ions can move freely:

• Do not conduct when solid: In solid ionic compounds, ions are fixed in place within the lattice and cannot move, so no electrical conductivity occurs.
• Conduct when molten or dissolved in water: When ionic compounds melt or dissolve in water, the lattice breaks down, freeing the ions. These free ions can move and carry electric charge, allowing the substance to conduct electricity.

For instance, molten sodium chloride conducts electricity because the Na⁺ and Cl^2 ions are free to move and carry charge.

Example: Sodium chloride does not conduct electricity in its solid form but does conduct when dissolved in water or molten.

Solubility

Solubility is the ability of a substance to dissolve in a solvent. Water is a polar solvent, meaning it has positive and negative ends, while non-polar solvents like hexane do not have charged ends.

Ionic compounds generally dissolve well in water but not in non-polar solvents:

• Generally soluble in water: Water's positive and negative ends attract the positive and negative ions in the ionic compound, pulling them apart and dissolving the compound.
• Ions separate in solution: When dissolved, the ionic lattice breaks apart and individual ions disperse throughout the water, allowing the solution to conduct electricity.
• Insoluble in non-polar solvents: Non-polar solvents like hexane do not have charged ends to attract ions, so ionic compounds do not dissolve well in them.

For example, potassium chloride dissolves easily in water but not in oil.

For instance, when sodium chloride dissolves in water, the Na⁺ and Cl^2 ions separate and disperse evenly throughout the solution.

Structure and Bonding

The properties of ionic compounds arise from their structure and bonding:

• Giant ionic lattice: Ionic compounds form a 3D lattice of alternating positive and negative ions. This lattice extends throughout the solid, creating a strong, stable structure.
• Strong electrostatic forces: The ions are held together by strong electrostatic forces of attraction between oppositely charged ions. These forces act in all directions, making the lattice very strong.
• Ions arranged in regular pattern: The ions are arranged in a repeating, regular pattern that maximises attraction and minimises repulsion, contributing to the compounds stability and high melting point.

Example: In sodium chloride, each Na⁺ ion is surrounded by six Cl^2 ions and vice versa, forming a cubic lattice.