1.4.7 Properties of Ionic Compounds

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Ionic compounds are made up of positively charged cations and negatively charged anions held together by strong electrostatic forces in a giant ionic lattice. These forces of attraction give ionic compounds several distinct physical properties.

Hardness and Brittleness

Hardness

Ionic compounds are typically hard because the anions and cations are strongly attracted to each other, forming a rigid structure.
This makes the ions difficult to separate.

Brittleness

Despite their hardness, ionic compounds are brittle.
When a force is applied to an ionic solid, ions of like charge may be forced closer together, causing electrostatic repulsion.
This repulsion can split the solid, making ionic compounds prone to breaking or shattering under stress.

Melting and Boiling Points

High Melting and Boiling Points

Ionic compounds have high melting and boiling points because of the strong electrostatic forces between the oppositely charged ions in the lattice.
A large amount of energy is required to overcome these forces and change the state of the compound from solid to liquid or gas.

Factors Affecting Melting Points

Smaller ions have stronger forces of attraction because they can pack more closely together.
Ions with higher charges (e.g., $Be²$ ⁺ and $O²⁻$ in BeO) form stronger bonds, leading to higher melting points compared to compounds with lower charged ions (e.g., $Na⁺$ and $Cl⁻$ in NaCl). For example, BeO has a melting point of 2,578°C, much higher than LiF, which melts at 848.2°C.

Electrical Conductivity

In Solid State

Ionic compounds do not conduct electricity in their solid state because the ions are fixed in place by strong ionic bonds and cannot move.

In Molten State or in Solution

Ionic compounds do conduct electricity when molten or dissolved in water, as the ions are free to move and carry charge.
For example, NaCl in water or molten NaCl will conduct electricity, while solid NaCl will not.

Solubility

Solubility in Polar Solvents

Ionic compounds are generally soluble in polar solvents like water. This is because water molecules can interact with the ions in the lattice, knocking ions off the outer layers, causing the compound to dissolve.

Insolubility in Certain Cases

Some ionic compounds, like aluminium oxide (Al₂O₃), have such strong electrostatic attractions that water does not have enough energy to break the lattice. This makes these compounds insoluble in water.