: 180
Fajans' rules predict covalent character in nominally ionic compounds. Covalent character increases when: (1) cation is small with high charge (high charge density polarises the anion), (2) anion is large (more polarisable electron cloud), (3) cation has pseudo-noble-gas configuration (d10 or d10s2, e.g., Cu+, Ag+, Zn2+, Tl+ — less effective nuclear shielding). Applications: LiI > LiF in covalent character (I- larger than F-). BeCl2 is covalent (tiny Be2+). AgCl is covalent despite being a halide (Ag+ has d10 configuration). Among alkali halides, LiI is most covalent, CsF is most ionic. High covalent character leads to: lower melting points, solubility in organic solvents rather than water, coloured compounds (polarisation affects electronic transitions). Lattice energy follows the opposite trend: most ionic compounds (small ions, high charges) have highest lattice energies.