Electronic charge, core-valence separation

When multi-electron atoms are combined to form a chemical bond they do not utilize all of their electrons. In general, one can separate the electrons of a given atom into inner-shell core electrons and the valence electrons which are available for chemical bonding. For example, the carbon atom has six electrons, two occupy the inner Is orbital, while the remaining four occupy the 2s and three 2p orbitals. These four can participate in the formation of chemical bonds. It is common practice in semi-empirical quantum mechanics to consider only the outer valence electrons and orbitals in the calculations and to replace the inner electrons + nuclear core with a screened nuclear charge. Thus, for carbon, we would only consider the 2s and 2p orbitals and the four electrons that occupy them and the +6 nuclear charge would be replaced with a +4 screened nuclear charge. 

Any given molecule has two centers of charge, one associated with the positive nuclei of the ion cores, the other associated with the negative valence electrons. For a spherically symmetric molecule (and others) these centers are coincident. When an electric field is applied to such a molecule (or to a solid containing such molecules), the centers of charge separate by some distance, x forming an electric dipole with a moment, p = qx where q is the amount of charge associated with each center, p and x are determined by the polarizability which will be considered later. 

Figure 12 Principle of charge separation in semiconductor heterostructures (a) capped (or Core-Shell) geometry and (b) coupled geometry. Electrons accumulate at the conduction band (CB) of Sn02 while holes accumulate at the valence band (VB) of Ti02.

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