Chemistry · Chemical Bonding · NEET
Fajans' rules are three conditions that make an ionic bond behave more like a covalent bond. Covalent character increases when: (1) the cation is small, (2) the anion is large, and (3) the charge on either ion is high. There is a fourth helper: a cation with a noble-gas (18-electron) outer shell polarises more than one with an 8-electron shell. When the positive ion pulls the negative ion's electron cloud strongly, the electrons get shared instead of fully transferred, so the bond gains covalent character.
A pure ionic bond would mean the cation takes the electron completely and the two ions just sit side by side. In real life the small, positive cation pulls back on the big electron cloud of the anion. This pulling (polarisation) drags shared electron density into the space between the ions. That shared density is exactly what a covalent bond is. So every ionic bond has some covalent character, and every covalent bond has some ionic character. This matters for NEET because it explains melting point and solubility trends.
Polarising power belongs to the CATION: it is how strongly the cation can distort (pull) an electron cloud. Small size and high positive charge give high polarising power (charge density = charge / size). Polarisability belongs to the ANION: it is how easily the anion's cloud gets distorted. Large size and high negative charge make an anion easily polarisable. High polarising power PLUS high polarisability equals maximum covalent character.
Li+ is the smallest cation in that group, so it has the highest polarising power (highest charge density). It pulls the Cl- cloud hardest, giving LiCl the most covalent character. As you go down the group (Na+, K+, Rb+, Cs+) the cation gets bigger, polarising power drops, and the compounds become more ionic. That is why LiCl dissolves in organic solvents and has a lower melting point than NaCl.
Al3+ carries a +3 charge and is very small, so it has a huge charge density and very high polarising power. It distorts the Cl- cloud strongly, giving AlCl3 lots of covalent character (it even sublimes and dissolves in organic solvents). Na+ is only +1 and larger, so its polarising power is low and NaCl stays highly ionic (high melting point, dissolves in water). Compare BeCl2 (covalent) vs CaCl2 (more ionic) the same way.
I- gives the most covalent character. Going F- to I-, the anion gets bigger, so its electron cloud is looser and easier to polarise. So for the same cation, the iodide is the most covalent and the fluoride is the most ionic. Example: AgF is the most ionic and AgI is the most covalent silver halide, which is why AgI is nearly insoluble in water.
A cation with an 18-electron outer shell (like Cu+, Ag+, Zn2+, Cd+, Hg2+, called pseudo-noble-gas type) polarises more than a cation of the same size and charge with an 8-electron (noble-gas) shell. The reason is that d-electrons shield the nucleus poorly, so the effective nuclear pull on the anion is stronger. That is why CuCl is more covalent than NaCl even though Cu+ and Na+ are similar in size.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Fajans' rules are used for ionic (electrovalent) compounds to judge how much covalent character they pick up. They are not needed for molecules that are already covalent. But the underlying idea, polarisation, still explains bond properties everywhere.
CuCl. Cu+ has an 18-electron (pseudo-noble-gas) outer shell that polarises the chloride cloud more strongly than the 8-electron Na+ of similar size. So CuCl is more covalent, even though both cations carry a +1 charge.
More covalent character means lower melting point and better solubility in non-polar (organic) solvents, plus lower solubility in water. So a more covalent compound like AlCl3 or AgI behaves very differently from a highly ionic compound like NaCl.
No. Every ionic bond has some covalent character due to polarisation, and every covalent bond between different atoms has some ionic character due to electronegativity difference. Fajans' rules describe the ionic-to-covalent side of this.
Small cation, Big anion, High charge equals more Covalent. For polarising power think 'charge density = charge over size', so a small highly charged cation wins.
