| Formulated by the Polish-born chemist Kasimir Fajans in 1923-4, these rules govern the degree of covalency present in ionic compounds.
Theoretically, the bonding in these compounds is purely ionic - an attraction between positive and negative ions. In practice, however, the electrons on the negative ion (the anion) can also be attracted towards the positive ion (the cation), distorting the shape of the anion and resulting in a degree of covalency (electron sharing). This makes the bonding in the compound stronger.
Fajans' rules state that there is a tendency towards increasing covalency when:
- the positive ion is small
- the positive ion has multiple charges
- the negative ion is large
- the negative ion has multiple charges.
Therefore the bonding in beryllium oxide (BeO) has a strong covalent character in addition to the ionic bonding, because the ions have a double charge and beryllium ions are also very small. Potassium fluoride (KF), on the other hand, has relatively little covalency, so its actual bond strength (lattice energy) is similar to its theoretical value.
Source: Michael Volkins (general editor), Nuffield Advanced Chemistry, 2000 | 
