S-p separation

Fig. 7.—Energy curves for the lithium molecule-ion, with consideration of the s-p separation.

Although the s-p separation is nearly twice as large as the bond energy, there occurs extensive hybridization of the bond orbitals at the equilibrium intemuclear distance the bond orbitals are composed nearly equally of s and Pp, the ratio b/a being 0.92 (Fig. 8). This hybridization increases the bond energy by more than 100%, from 0.54 e. v. for a pure s bond (Fig. 7) to 1.19 e. v. The contribution of bond energy are of course smaller than they would be for zero s-p separation (b/a = 1.9 and D = 2.16 e. v. at r = 3.0 A.). It may be pointed out... 

The energy of the one-electron bond in the lithium molecule ion is calculated with consideration of the s-p separation to be 1.19 e. v and the hybrid bond orbital involved is shown to involve about equal contributions from the 25 and 2p orbitals of the lithium atom. 

Electrons near the Fermi level are mobile and give rise to electrical conductivity. The band formed from overlap of p atomic orbitals is called the p band. The overlap of andp orbitals produces andp bands. If separation between andp atomic orbitals is large, the separation between s and p bands in metal is large. There is a bandgap in this case (Fig. 3.11). When the s-p separation is smaller, the bands overlap. 

The number of states within each band is equal to the total number of orbitals contributed by the atoms. For one mole of metal, a s band will consist of No states and a 3p band of 3No states. These bands will remain distinct if the s-p separation is large, as shown in Fig. 4.3.3(a). However, if the s-p separation is small, the s and p bands overlap extensively and band mixing occurs. This situation, as illustrated in Fig. 4.3.3(b), applies to the alkaline-earth metals and main group metals. 

Now let us return to the formation of F2. The valence orbitals of F are the 2s and 2p, and Figure 1.21 shows a general orbital interaction diagram for the overlap of these orbitals. We may assume to a first approximation that the energy separation of the fluorine 2s and 2p atomic orbitals (the s-p separation) is sufficiently great that only 2s-2s and 2p-2p orbital interactions occur. Notice that the stabilization of the Ttu 2px) and TT 2py) MOs relative to the 2p atomic orbitals is less than that of the Gg 2pz) MO, consistent with the relative... 

Fig. 1.21 A general orbital interaction diagram for the formation of X2 in which the valence orbitals of atom X are the 2s and Ip. In constructing this diagram we assume that the s-p separation is sufficiently large that no orbital mixing occurs. The X nuclei lie on the z axis.

The [HF2] ion (see Figure 9.8) has Z) i, symmetry and the axis coincides with the axis. The bonding in [HF2] can be described in terms of the interactions of the H li orbital ag symmetry) with the LGOs of an F F fragment. If we assume a relatively large s—p separation for fluorine, then sets of LGOs can be constructed as follows ... 

Barber, E. D., Sawicki, E., and McPherson, S. P., "Separation and Identification of Phenols in Automobile Exhaust by Paper and Gas Liquid Chromatography," Analytical Chemistry, 36, 2442, 1964. 

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