Fajans’ rule

Fajans rules Ionic compounds are most readily formed by ... 

A number of trends connected with ionic radii are noticeable across the series. In keeping with Fajans rules, salts become somewhat less ionic as the Ln " radius decreases reduced ionic character in the hydroxide implies a reduction in basic properties and, at the end of the series, Yb(OH)3 and Lu(OH)3, though undoubtedly mainly basic, can with difficulty be made to dissolve in hot cone NaOH. Paralleling this change, the [Ln(H20)j ] + ions are subject to an increasing tendency to hydrolyse, and hydrolysis can only be prevented by use of increasingly acidic solutions. 

Fajans rules state that the extent of polarization of an anion or the partial covalent character of an otherwise 100% ionic compound is increased by ... 

These are Fajans rules (1924) they were originally empirical deductions. 

A Second Look In the usual discussion of Fajans rules as given in Chapter 4, emphasis is placed on... 

However, even the simple 1 ion is highly polarisable. This is a well accepted fact in other branches of chemistry. Fajans Rules in inorganic chemistry deal explicitly with this effect in bonding. In summary ... 

The latter approach was championed by Fajans and is embodied in Fajans rules, whose basic premise is summarized in Fig. 4.2. In Fig. 4.2a an idealized ion pair is shown for which the covalent character is nonexistent (i.e., the ions are assumed to be hard spheres). In Fig. 4.2 > some covalent character is imparted by shifting the electron cloud of the more polarizable anion toward the polarizing cation. In the extreme case that the cation is totally embedded in the electron cloud of the anion (Fig. 4.2c) a strong covalent bond is formed. The extent to which the electron cloud is distorted and shared between the two ions is thus a measure of the covalent character of that bond. The covalent character thus defined depends on three factors ... 

The acidity of M will decrease within any group with increasing atomic volume (effectively, with increasing atomic number) owing to the weaker attraction between nuclear charge and incoming electron pairs (Fajans rules).The result of these effects leads to the order ... 

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