Fajans' RulesJEE Main
When ionic bonds go covalent — interactive Chemistry simulation for IIT-JEE.
Concept
No bond is purely ionic. A small, highly charged cation pulls the electron cloud of a large, squishy anion toward itself — the bond gains covalent character. Fajans' rules: covalency grows with cation charge, shrinking cation size, and growing anion size/charge. That's why AgI is yellow-ish and insoluble while NaF is a textbook ionic salt.
Key formula
Derivation
The cation's electric field at the anion's surface scales as q/r². The anion's cloud distorts in proportion to its volume (r³) and looseness (charge).
Distorted cloud = shared density between the nuclei = partial covalent bond. Pseudo-noble-gas cations (Cu⁺, Ag⁺, Zn²⁺ — 18-electron shells) polarize extra strongly for their size.
Scenarios to explore
- Fajans' Rules — Polarization turns ionic bonds covalent — AgI vs NaF.
Real-world applications
- Solubility ladder AgF > AgCl > AgBr > AgI (rising covalency).
- Melting points: NaCl 801 °C vs AlCl₃ 192 °C (covalent network vs molecular).
- Colour deepening: AgCl white → AgI yellow (charge-transfer eased by polarization).
JEE exam tips
- LiCl covalent enough for alcohol solubility; NaCl isn't — classic contrast.
- BeCl₂: Be²⁺ tiny + 2 charge → largely covalent, bridged polymer solid.
- Ionic character order for AlX₃: F > Cl > Br > I — bigger halide, more covalent.
Common mistakes
- Ranking by charge alone — SIZE matters as much (Li⁺ beats Na⁺).
- Forgetting 18-electron cations (Ag⁺, Cu⁺) out-polarize same-size noble-gas cations.
- Confusing polarizing POWER (cation's) with POLARIZABILITY (anion's).
Exam traps to avoid
- More covalent character usually means LOWER melting point (molecular lattice) — the counter-intuitive one.
- Fajans complements electronegativity: use Δχ for element pairs, Fajans for ion pairs.