9. Molecular Orbitals and      Molecular Structure   Previous PageNext Page
     Larger Diatomic Molecules

As with H, , and , we can establish the number of bonds between atoms in these molecules by counting the net number of bonding electrons and dividing by two, since we are accustomed to calling two bonding electrons a "bond." Li, found when lithium metal is vaporized, has a single bond. Be has no net bonding for the same reason He. has none, so vaporized Be consists of single atoms. B has a single bond, C a double bond, N and CO triple bonds, and 0 a double bond because of the disruptive effect of the and antibonding electrons. F has only a single bond, and Ne has no bond at all, like He and Be . These predictions about bond order from MO theory correspond quite well with the observed bond lengths and bond energies, which are tabulated below the filling diagrams. Bond lengths decrease and bond energies increase with increasing bond order from B to N, and then reverse their trends as the bond order decreases from N to Ne. N and CO have the same bond structure and show similar bond lengths and bond energies. The single-double- triple-double-single sequence of bond orders for B through F is entirely the consequence of the order of energies of the MO's as diagramed at the left. If this sequence of energy levels had been different-if all the bonding MO's had come before the antilionding, for example-then the predictions of bonding in diatomic molecules by MO theory would have been quite wrong.

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