Fajans' rules

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In inorganic chemistry, Fajans' Rules, formulated by Kasimir Fajans, are used to predict whether a chemical bond will be covalent or ionic, and depend on the charge on the cation and the relative sizes of the cation and anion. They can be summarized in the following table:

Ionic	Covalent
Low positive charge	High positive charge
Large cation	Small cation
Small anion	Large anion

Thus sodium chloride (with a low positive charge (+1), a fairly large cation and small anion) is ionic; but aluminium iodide (AlI₃) (with a high positive charge (+3) and a large anion) is covalent.

Polarization will be increased by:

• High charge and small size of the cation
  • Ionic potential Å Z+/r+ (= polarizing power)
• High charge and large size of the anion
  • The polarizability of an anion is related to the deformability of its electron cloud (i.e. its "softness")
• An incomplete valence shell electron configuration
  • Noble gas configuration of the cation produces better shielding and less polarizing power
    • e.g. Hg²⁺ (r+ = 102 pm) is more polarising than Ca²⁺ (r+ = 100 pm)