Ionic charge, formal

Where it is not feasible to define an oxidation state for each individual member of a group, the overall oxidation level of the group is defined by a formal ionic charge to avoid the use of fractional oxidation states for example, Oy. 

Many of the tertiary bonds reported by Preiser et al. (1999) are likely artefacts of their calculations since these were based on the use of formal ionic charges. Substituting a more physically reasonable value for the formal ionic charge will reduce the total flux starting at the cation and eliminate many of the tertiary bonds around the highly charged cations where most tertiary bonds were found. However, there are some cases where tertiary bonds undoubtedly do occur and these can provide important information about the crystal chemistry. 

Condition 3.1. The stoichiometry must obey the electroneutrality principle, namely that the sum of all the atomic valences (formal ionic charges), having regard to their sign, is zero. 

The number, 0.75 vu, is sometimes called the formal charge on the O atoms, but it should not be confused with either the formal ionic charge (=-2 for all ions) or the charges on the O atoms calculated by quantum mechanics. Quantum mechanical charges are usually larger than -0,75 (depending on how the calculation is performed) since they include ionic contributions to the P-O bonds as well as to the external bonds. Quantum mechanics does not allow one to separate the internal and external bond contributions. 

One can calculate cation bonding strengths, s, in the same way as anion bonding strengths by dividing the valence (or formal ionic charge) of a cation by its ideal coordination number, (eqn (4.2)) ... 

Nonionic. The head group bears no formal ionic charge. 

Figure 34 Phase diagram showing the dependence of B-site ordering on the differences in formal charge and ionic radii in A2B B"Og perovskites. (Reprinted from Ref. 78, 1993, with permission from Elsevier)...

According to Gauss electrostatic theorem, the total bond flux, Sij, leaving the core of atom i is equal to its charge, which is the same as the number of valence electrons, (the formal ionic charge see Formal Charge) or atomic valence), that the atom uses in bonding, equation (2). 

The simple electrostatic model of hydrolysis (Baes and Mesmer 1976), which predicts log K to change linearly with the ratio of the ionic charge to ionic radius, does not explain, for example, the difference between Nb and Ta having the same value of the formal charge and ionic radius, as it does not explain the reversed order... 

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