Charge excess

In a high dielectric constant medium (i.e. water), when a potential gradient is established between an electrode and the solution a charge excess will develop on the electrode surface which must be balanced by solution species. Solvent molecules and other species (i.e. electrolyte) orient themselves at the electrode surface to counter the elec-... 

Fig. 4.6 The adsorbed anion A induces a positive charge excess at the outer Helmholtz plane, which can be represented as its image A. The plane of imaging is identical with the outer Helmholtz plane. (According to B. B. Damaskin, O. A. Petrii and V. V. Batrakov)...

FIGURE 14. (a) Representation of charge excess, Q, and defect, 0> on the four carbon atoms for the two orbitals n. (left) and tr (right) for 1,3-butadiene, (b) Electric transition moments arising from the 7T - n transition... 

Fig. 10.5 Helmholtz model of the interface between a metal and an electrolyte. The metal is shown with a negative charge (excess of electrons) which is balanced by an excess of mobile cations, the centres of which are one atomic radius from the surface.

The charge excess in solution, qs, is related with the surface excess of cations and anions in solution,... 

On the surface the charge excess of an ion is defined as the formal charge Q of the ion considered plus the sum of the Pauling valencies, computed with respect to the other ion ... 

A negative value is computed for e on the oxygen anion. A positive charge excess implies Lewis acidity and a negative charge excess implies Lewis basicity. When the surface becomes exposed to NH3, ammonia will be adsorbed to the Mg2+ ion. 

To determine acidity or basicity it is useful to compute the charge excesses on oxygen atoms 0(1) and 0(2). 

As we saw earlier when we discussed the acidity of the silanol group, computation of charge excesses provides only a very crude estimate of acidity. As we will see for an explanation of differences in the Bronsted acidity of different zeolites, Pauling s valency rule is too crude. 

Surface Lewis acidic or basic sites are created on ionic surfaces because surface atoms have a lower coordination number compaired to their bulk values. A condition for surface stability is that the sum of the charges over the surface unit cell is zero and the surface is overall neutral. A useful method to estimate the degree of coordinative unsaturation on an ionic surface is to use Pauling valencies and to compute charge excesses. The Pauling valency, v, is defined as ... 

On the surface the charge excess of an ion is defined as the formal charge Q of the... 

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With modem data the electronegativity values can be used to estimate the charge transfer (8 or AN/2 AN, differential charge of dipole) in diatomic molecules, as illustrated in Table 3-2 for HCl, metal-chlorine molecules, and metal-oxygen molecules. The charge excesses (5) for the chlorine and oxygen of several diatomic molecules [estimated on the basis of their respective... 

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