Ionic solids in solution

Ionic solids exist as lattices in which the cation is surrounded by anions and the anion is surrounded by cations.

A portion of the lattice for NaCl is shown at the right.

The attraction of the anions for the cations is referred to as ionic bonding. Ionic bonds are strong, and the melting point of ionic solids is high (> 500°C) because significant energy must be added to the system to enable the cation and anion to move freely in a melt.

Despite the strong attractive forces between the ions, NaCl dissolves readily in water at room temperature. Why is this so?

Water is a polar solvent.
In water molecules, the OH bonds are polar covalent.

Hover over the diagram to see a charge map.
The partial negative charge (δ^–) is at O and partial postive charge (δ⁺) at H.

The water molecule is polar because both of the hydrogens bonded to the oxygen are on one side of the molecule, and that portion of the molecule is more negative.

Interaction of ions with the polar solvent water
Ionic solids dissolve in water if the strength of the attractive force between the ions and the water molecules is comparable to the strength of the ionic bonds in the lattice of the ionic solid.

The diagrams show the nature of the interaction of water molecules with the ions.
Note that the orientation of the water molecules depends on ion charge.

For cations (positive ions) the partially negative oxygen is closest to the ion.
For anions (negative ions) the partially positive hydrogens iare closest to the ion.

Complexes of ions with water are known as hydrated ions and may be written as M⁺(aq).or X^–(aq).

Note that in an actual solution there are more than four water molecules around each one as there are water molecules both behind and in front of the ions shown in the diagram.