Electronic configuration Fajans

Thus increased covalent bonding resulting from Fajans-type phenomena can lower the transition temperatures. For example, the alkali halides (except CsCl, CsBr, and Csl) and the silver halides (except Agl) crystallize in the NaCl structure. The sizes of the cations are comparable Na = 116 pm, Ag = 129 pm, K = 152 pm, yet the melting points of the halides are considerably different (Table 8.6). The greater covalent character of the silver halide bond (resulting from the electron configuration) compared with those in the alkali halides helps stabilize discrete AgX molecules in the liquid and thus makes the melting points of the silver compounds lower than those of the potassium compounds. A similar comparison can be made between the... 

Fajan s rules dictate that the partial covalent nature of a metal s ionic bond with an anion will increase with high charge of either of the interacting ions, small cation radius, or large anion radius. The covalent nature of the bond also is higher with a noninert gas electron configuration of the cation, e.g., the more covalent nature of Cu + ( Ar 3d ) bonds, in contrast to the ionic bonds formed by Na+ ( Ne ) (Barrett 2002). 

The term quantization or quantum configuration means here the kind of subdivision of the electronic system into definite groups (quanticules). For instance, Na+Cl- contains two mononuclear quanticules 1 2 and 1 2 3 , while H+ejH+ contains one binuclear quanticule e. See Fajans, K. Chimia 13, 349 (1959). Lithoprinted English translation at Ulrich s Book Store, Ann Arbor, Michigan. 

Uf all the different types of atomic aggregates, ionic crystals have been found to be most suited to simple theoretical treatment. The theory of the structure of ionic crystals described briefly in the following sections was developed about 40 years ago by Born, Haber, Land6, Madelung, Ewald, Fajans, and other investigators. The simplicity of the theory is due in part to the importance in the interionic interactions of the well-understood Coulomb terms and in part to the spherical symmetry of the electron distributions of the ions with noble-gas configurations. 

Fajan s third rule states that covalent character is greater for cations with a nonrare gas configuration than for those with a complete octet, charge and size being approximately equal. For example, Cu is more polarizing than Na (ions of approximately the same size) because the d electrons around Cu do not shield the nucleus as effectively as the complete octet in Na. ... 

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