9. Molecular Orbitals and      Molecular Structure   Previous PageNext Page
     Bonds between different kinds of atoms

The bond line represents the bonding orbital electrons, and the three lone pairs around F are in the 2s, 2p and 2p orbitals. This treatment of HF illustrates how two different quantum levels on different atoms can be combined if they have similar energies. It also illustrates the effect of difference in electronegativities on the character of the chemical bond. The bonding MO in HF is closer in energy to the 2p AO of fluorine from which it came, and less like the 1s AO of hydrogen. The antibonding MO, in contrast, is closer in energy to the hydrogen 1s. If you worked out the mathematics of combining AO's into MO's, you would find that the bonding MO has a greater contribution from the fluorine 2p and the antibonding MO is more like the hydrogen 1s orbital. In H, of course, the two atoms are identical, and the bonding and antibonding MO's have equal contributions from both atoms.
The lower energy of the fluorine 2p AO, compared with that of the hydrogen 1s, is a reflection of the fact that F is more electronegative than H, and holds onto its outer electrons more tightly. In general, when two atoms of different electronegativities are combined, the bonding MO's are more like the AO's of the more electronegative element in both shape and energy, and the antibonding MO's resemble those of the less electronegative atom.

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