Fajans' rules predict when an "ionic" compound develops significant covalent character. Covalent character increases with: (1) small cation size (high charge density polarises anion), (2) large anion size (more polarisable electron cloud), (3) high charge on cation or anion, (4) pseudo-noble-gas configuration of cation (d10 or d10s2, e.g., Cu+, Ag+, Tl+ — less effective nuclear shielding means more polarising power). Applications: LiF is most ionic among LiX; LiI is most covalent. BeCl2 is covalent (small Be2+, high charge density); BaCl2 is ionic (large Ba2+). AgCl is covalent (Ag+ has d10 configuration) despite being a halide.