Q.State Fajans' rules for predicting the covalent character in an otherwise ionic bond. Using these rules, explain why lithium chloride ($\text{LiCl}$) shows appreciably more covalent character than sodium chloride ($\text{NaCl}$).

Real chemical bonds are rarely purely ionic or purely covalent — both are idealised limiting cases, and the great majority of actual bonds lie somewhere on a continuum between complete electron transfer and perfectly equal electron sharing. Fajans' rules address one end of this continuum: they predict when a bond that is nominally ionic will, in reality, acquire significant covalent character.

The physical mechanism behind this is polarisation. A cation's concentrated positive charge can attract and distort the electron cloud of a neighbouring anion, pulling some of that electron density back toward itself. To the extent that this happens, electron density becomes genuinely shared between the two ions rather than fully transferred, which is exactly what covalent bonding means — so a strongly polarised "ionic" bond behaves, in practice, as a partially covalent one. Kazimierz Fajans formalised the conditions under which this polarisation, and hence covalent character, is most pronounced, generally stated as three rules:

1. Small, highly charged cations polarise more strongly. A small cation concentrates its positive charge over a small volume, producing an intense local electric field; a highly charged cation (e.g. $2+$ or $3+$ rather than $1+$) has an even stronger field for the same reason.
2. Large, highly charged anions are more easily polarised. A large anion holds its outermost electrons more loosely and further from its own nucleus, making that electron cloud easier for a neighbouring cation to distort; a highly charged anion is similarly more loosely bound and more easily distorted. …