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

These look so much like slightly larger versions of the H orbitals that it is unnecessary to draw them again. If the bond axis is chosen as the z axis, then the two 2p orbitals can be combined, as in the left drawing, to yield a bonding orbital and an antibonding orbital. The MO's are both s, orbitals because they are symmetrical about the bond axis. Each of the original p orbitals had one nodal plane of zero electron probability, and these nodes are preserved in the resulting bonding MO. When we bring density lobes of opposite signs together in making the antibonding MO, we introduce another change of sign, so the antibonding orbital has three nodal surfaces, as shown. The bonding is like the in concentrating most of the electron probability between the nuclei, where it helps to hold the atoms together. The antibonding resembles the in having most of the density on the outside of the nuclei, where it tends to pull the atoms apart.

The 2p and 2p atomic orbitals are at right angles to the bond line, and can be paired to make bonding and antibonding MO's as in the lower figure on the preceding page. Only p orbitals are shown, but the p orbitals look just like them, at right angles out of the page. These AO's lead to p MO's, with a change of sign upon 180' rotation around the bond axis. The bonding and orbitals preserve the nodal surface present in the original p and porbitals, and the antibonding and add an extra node that is introduced when the p's are combined with opposite signs. }

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