Fajans' rules
/ FAY-yahnz /
We say NaCl is ionic and CCl4 is covalent, but how ionic is a compound really? Fajans' rules are a quick set of guidelines, from the 1920s, for judging how much covalent character a nominally ionic compound has. The idea is that a positive ion can distort the electron cloud of a neighbouring negative ion, pulling some of its electrons into the space between them — and shared electrons mean covalent character.
The rules say covalent character increases when the cation is small and highly charged (it has a strong, concentrated pull, high polarising power), when the anion is large and highly charged (its loose outer electrons are easily distorted, high polarisability), and when the cation does not have a noble-gas electron configuration (a d10 cation like Ag+ or Cu+ shields its nucleus poorly, so it polarises more strongly than an s-block cation of the same size and charge). In short: small, highly charged, non-noble-gas cations plus big, soft anions equal more covalent, less ionic bonding.
These rules neatly rationalise trends that pure ionic thinking gets wrong: why AgCl is far less soluble and more covalent than NaCl despite both being 1+/1- salts; why AlCl3 sublimes as covalent molecules while NaCl is a giant ionic lattice; why lithium and beryllium compounds are anomalously covalent for the s-block. They are qualitative rules of thumb, not a calculation, and they capture the same physics as the more general idea of ion polarization.
Across AlF3, AlCl3, AlBr3, AlI3 the cation Al3+ is fixed but the anion grows softer; iodide is the most polarisable, so AlI3 has the most covalent character and the lowest melting point, while AlF3 is the most ionic. Fajans' rules predict exactly this order.
Same cation, increasingly polarisable anion: covalent character rises down the halide series.
Fajans' rules are qualitative trends, not predictions of a number; they tell you the direction of covalent character, not its exact value, and they are closely related to the hard-soft acid-base idea.