Ionic Compounds

Ionic compounds are made up of cations and anions, held together in a 3-dimensional lattice by strong, directional ionic bonds. However, not all ionic bonds are equal...

Smaller ions have stronger ionic bonds than larger ions (of the same charge).

Ions with a higher charge have stronger bonds than ions with a lower charge (of a similar radius).

We need to consider these two factors (together), when inferring the relative strength of ionic bonds.

The strength of an ionic bond can be inferred from a substance's melting point, boiling point and/or solubility in water. 

The higher the m.p./b.p., the stronger the ionic bond. This is because melting and boiling break ionic bonds to turn the substance into liquids or gases, respectively.

The higher the solubility in water, the weaker the ionic bond. This is because dissolving requires the ions to have a strong enough attraction to the water molecules to break the ionic bonds holding them in the ionic lattice.

More Detail...

If you want to know more about this, and the link it has to Coulomb's Law, check out this video. It is a bit of "extra for experts" and is not assessed in Level 3 NCEA.