9. Molecular Orbitals and      Molecular Structure   Previous PageNext Page
     Hydrogenlike Molecules

This bonding-antibonding arithmetic is summarized in the table on the previous page. We can calculate the number of electron-pair bonds by counting the net number of bonding electrons and dividing by two. If we refer to an electron-pair bond as a "single bond" then H has a single bond, and have one half bonds, and He has no bond at all. The number of bonds between two atoms in a molecule is termed its bond order: and have bond order 1/2, H has bond order 1, and the C-C double bond in ethylene has bond order 2. Bond order is related to bond length and bond energy. The measured experimental bond lengths, r, and bond energies, BE, are given below the filling diagrams. The single bond in H is 0.74 A long, and 103 kcal moleof energy is required to break the bonds and turn a mole of H molecules into H atoms. The half bonds of the two molecule-ions re- quire approximately half as much energy to break them, and are weak enough to allow the bond length to stretch from 0.74 Å to a little over I Å. Simple MO theory is correct in predicting trends in bond lengths and bond energies for these small molecules, and this should give us confidence that the basic approach to bonding is correct.

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