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By Root 430 0

say we bring the H atom and the F atom toward each other along the z axis as shown at the top of Figure 13.10. The two circles reflect the correct relative sizes of the H and F atoms. Fluorine’s 2py orbital has its lobes perpendicular to the z axis, as shown in the middle portion of the figure. (The orbitals are not drawn to scale.) When the 2py overlaps the hydrogen 1s orbital, the positive 2py lobe will constructively interfere with the 1s orbital, but the negative lobe will destructively interfere. The net result is that there is no net constructive or destructive interference. The same is true for the 2px orbital. The 2py and 2px orbitals will not form bonding or antibonding MOs in the HF molecule. The lower portion of the figure shows the 2pz orbital’s positive lobe overlapping with the 1s orbital, which is also taken to be positive. This overlap will result in constructive interference of the probability amplitude waves, and can produce a bonding MO. Since there is electron density along the line connecting the nuclei, the bond will be a σ bond. If the F negative 2pz lobe overlaps with the H positive 1s lobe, there will be destructive interference that gives rise to an antibonding MO.

FIGURE 13.10. Top: H and F atoms brought together along the z axis. Circles show the relative size of the atoms. Middle: overlap of the H 1s and F 2pyorbitals. There is equal constructive (+) and destructive (-) interference in the overlap region. No MO formation. Bottom: overlap of the H 1s and the F 2pzorbitals. There is constructive interference in the overlap region.

As discussed above, the fluorine 1s orbital energy is so much lower than the hydrogen 1s, that the fluorine 1s electrons do not participate in bonding. The outermost electrons in an atom, that is, the last shell that is filled, are the ones that contribute to bonding. These are the valence electrons. For elements in the second row of the Periodic Table, like fluorine, the 2s and 2p are the valence electrons. In making molecular orbital energy level diagrams, usually only the orbitals associated with the valence electrons are shown because these are the orbitals that can be involved in bonding. Figure 13.11 shows the MO energy level diagram for HF with the F 1s orbital and electrons left out. The energy level spacings are not to scale. As discussed in connection with Figure 13.10, the H 1s atomic orbital will combine with the F 2pz atomic orbital to form bonding (σb) and antibonding (σ*) MOs. This is indicated in the diagram by the dashed lines. This diagram is similar to the energy level diagram in Figure 13.5 except that now the atomic orbitals that form the MOs do not have identical energies.

Fluorine has nine electrons. Two are in the 1s orbital, which leaves seven. Hydrogen has one electron. So there are a total of eight valence electrons to place in the MO energy levels. The first two go into the level labeled 2s. The fluorine 2s orbital is much lower in energy than the hydrogen 1s, and these electrons do not participate in bonding. Therefore, the 2s molecular orbital is essentially the same as the fluorine 2s atomic orbital. The two electrons in the 2s orbital are a lone pair. The next two electrons go into the σb bonding MO. The final four electrons go into the 2px and 2py orbitals. Again, these are basically atomic orbitals of fluorine. They do not play a role in the bonding. These four electrons comprise two more lone pairs. While the lone pairs do not play a role in bonding, in polyatomic molecules they influence the shapes of molecules, which will be discussed in Chapter 14. The net result is that there are two electrons in the bonding MO and none in an antibonding MO. Therefore, HF has a single bond. Hydrogen and fluorine share a pair of electrons in the bonding MO. For H, the sharing provides the additional electron necessary to achieve the helium rare gas configuration. For F, the sharing provides the extra electron needed to obtain the neon rare gas configuration.

FIGURE 13.11. Molecular orbital energy level diagram for HF. The atomic orbitals