Fajans’ rules, formulated by American chemist of Polish origin. Kazimierz Fajans (1887-1975), indicating the extent to which an ionic bond has covalent character caused by polarisation of the ions. Covalent character is more likely if:
1. the charge of the ions is high;
2. the positive ion is small or the negative ion is large;
3. the positive ion has an outer electron configuration that is not a noble- gas configuration.