Fajans' Rules
A set of guidelines predicting how much covalent character an ionic bond will have.
Unit
Dimensionless
In depth
Covalent character increases with a small, highly charged cation, a large, highly charged anion, and a cation with a non-noble-gas configuration such as a d¹⁰ ion. These conditions maximise the cation's ability to distort the anion's electron cloud, pulling electron density into the internuclear region.
Examples in the real world
AlCl₃ is largely covalent and sublimes at 180 °C, while NaCl is ionic and melts at 801 °C; AgI is far more covalent than KI.