Covalent character in ionic bonds increases with small highly charged cations and large polarizable anions.
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Fajans' rules describe when an ionic bond acquires significant covalent character through polarization of the anion by the cation. The cation distorts the anion's electron cloud, pulling electron density toward the space between the nuclei. Polarization is strongest when the cation is small and highly charged, the anion is large and easily polarized, and the cation has a non-noble-gas electron configuration such as d10. Added covalent character lowers lattice stability relative to a purely ionic model, which often makes the salt less soluble in water. The rules are qualitative guidelines rather than exact laws.
Key ideas: Use them to predict deviations from ionic behaviour: more covalent character with high cation charge, small cation size, large anion size, and a pseudo-noble-gas cation (d10, such as Ag+, Cu+ or Zn2+). Example trend of covalent character in halides: LiF below LiI, and NaCl below AlCl3. Consequences include lower melting point and colour.
Common mistakes and connections: A typical error is to say that all salts are alike once ions form. Also forgotten is that high charge on the anion raises polarization too, so sulfide is more polarized than chloride. Colour in AgI or PbI2 arises from charge transfer linked to polarization. Connects to polarizability, lattice energy, solubility and melting points of halides.
Chlorides with more highly charged, smaller cations polarize Cl- more, so covalent character rises and melting points collapse from NaCl to SiCl4. The interactive lesson shows this as a live animation or chart.
Silver iodide is more covalent and less soluble than sodium iodide, because Ag+ is a strongly polarizing d10 cation and iodide is very polarizable.
Covalent character in ionic bonds increases with small highly charged cations and large polarizable anions.
Fajans' Rules is a advanced topic in Bonding & Molecular Structure. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Bonding & Molecular Structure first: Dipole Moment, Polar and Nonpolar Molecules, Percent Ionic Character.
The next topics in Bonding & Molecular Structure are Polarizability and Lattice Energy.
Yes. The interactive lesson includes a self-check question on Fajans' Rules, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Fajans' Rules is topic 25 of 104 in Bonding & Molecular Structure.