CHEM 121 glossary

Molecular orbital theory

A model of bonding in which the atomic orbitals of all the atoms combine into molecular orbitals spread over the whole molecule, and the electrons fill those MOs from the lowest energy up. Bond order and magnetism are read straight from the filling, including cases that Lewis structures and valence bond theory get wrong.

\ce{O}

atom

\ce{O2}

MOs

\ce{O}

atom

$\ce{O2}$ : one electron in each $\pi^*_{2p}$ MO, so 2 unpaired electrons.

Example

$\ce{O2}$ has 12 valence electrons; the last two go one each into the two degenerate $\pi^*_{2p}$ MOs, so MO theory predicts 2 unpaired electrons and a paramagnetic molecule, which is why liquid oxygen sticks to a magnet. A Lewis structure, with every electron paired, predicts diamagnetic.