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Fajans rules)
In inorganic chemistry, what became know as Fajans' Rules were formulated by Kasimir Fajans.
They are used to predict whether a chemical bond will be covalent or ionic, and depend on the charge on the cation and the relative sizes of the cation and anion. They can be summed up in the following table . . .
| Ionic |
Covalent |
| Low positive charge |
High positive charge |
| Large cation |
Small cation |
| Small anion |
Large anion |
Thus sodium chloride (with a low positive charge (+1), a fairly large cation and small anion) is ionic; but Aluminium Iodide (AlI3) (with a high positive charge (+3) and a large anion) is covalent.
