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Fajans' rules

    In inorganic chemistry '''Fajans' Rules''' formulated by Kazimierz Fajans in 1923 are used to predict whether a chemical bond will be covalent bond|covalent or ionic bond|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:
    IonicCovalent
    Low positive chargeHigh positive charge
    Large cationSmall cation
    Small anionLarge anion

    Thus sodium chloride (with a low positive charge (+1) a fairly large cation (~1 Å) and relatively small anion (2Å) 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
    • An incomplete valence shell electron configuration
      • Noble gas configuration of the cation produces better shielding and less polarizing power
        • eg Hg2+ (r+ = 102 pm) is more polarizing than Ca2+ (r+ = 100 pm)

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