Rules indicating the extent to which an ionic bond has covalent character caused by polarization of the ions. Covalent character is more likely if:

1. (1) the charge of the ions is high;

2. (2) the positive ion is small or the negative ion is large;

3. (3) the positive ion has an outer electron configuration that is not a noble-gas configuration.

The rules were introduced by the Polish-born US chemist Kasimir Fajans (1887–1975) in 1923.