Fractional ionic character

At present, the most widely used definition of fractional ionic character in solid state chemistry is that of Phillips (1970), based on a spectroscopic approach. Phillips defined fractional ionic character as... 

However, starting from the same experimental evidence, Stewart et al. (1980) adopt the following definition of fractional ionic character ... 

According to Stewart et al. (1980), the fractional ionic character thus defined is in agreement with the value deduced from electronegativity theory for molecular orbitals. The energy of a generic atom A is defined as... 

According to the treatment of Hinze et al. (1963), the charge transfer obeys the equality principle outlined in equation 1.31 (section 1.7). The transfer of fractional charges a and for two atoms A and B results in a stable bond with a fractional ionic character ... 

Summarizing, it suffices to mention that, instead of these ambiguities to which the concept of a fractional ionic character of a resonance bond led in the case of the molecules CsF, SrO, BaO, the conclusion of the quanticule-polarization interpretation reads as follows the quantum configuration is Cs+F-, Sr +0 , Ba +0 , the degree of polarity is 0.699, 0.483, 0.426 respectively. 

A large number of binary AB compounds formed by elements of groups IIIA and VA or IIA and VIA (the so-called III-V and II-VI compounds) also fcrystallize in diamond-like structures. Among the I-VII compounds, copper (I) halides and Agl crystallize in this structure. Unlike in diamond, the bonds in such binary compounds are not entirely covalent because of the difference in electronegativity between the constituent atoms. This can be understood in terms of the fractional ionic character or ionicity of bonds in these crystals. 

Here/j is the fractional ionic character of the bond while Xf and X are the Pauling electronegativities of the elements A and B. It is seen that/- = 0 when = Xg and fi= I whenX — Vg 1. In crystals, this expression is modified to account for the fact that an atom forms more than one bond and the number of bonds formed is not always equal to its formal valence. For crystals, the expression for ionicity reads as... 

The fractional ionic character, i, of such a bond is given by (1 - a) according to whether A is the least or most electro-negative partner. Thus... 

An empirical relation between the electronegativity difference Ax and the fractional ionic character of a covalent bond was suggested by Hannay and Smythe in 1946 ... 

For a diatomic molecule, the fractional ionic character can be defined by... 

The fraction ionic character of a bond between elements A and B can be approximated by... 

The two equations predict comparable fractional ionic character for most atom pairs, although the latter equation correlates better with the experimental result that the FI—F bond is 43% ionic and may also serve better for atoms with larger differences in electronegativity. ... 

Using this expression, compute the fractional ionic character for the following compounds ... 

The network former cations, such as Si + and B + that bond to oxygen with a fractional ionic character, form tetrahedra, whereas network modifier cations with very low electronegativities and therefore forming highly ionic... 

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