CHEM 121 glossary

Molecular orbital (MO)

An orbital that spreads over the whole molecule instead of belonging to one atom, formed by combining the atomic orbitals of the atoms (LCAO). Combining $n$ atomic orbitals always gives $n$ MOs; each MO holds at most two electrons of opposite spin, and they fill from the lowest energy up.

\ce{H}

atom

\ce{H2}

MOs

\ce{H}

atom

$\ce{H2}$ : both electrons sit in the bonding $\sigma_{1s}$ MO.

Example

In $\ce{H2}$ the two H $1s$ orbitals give two MOs: $\sigma_{1s}$ (bonding, holds both electrons) and $\sigma^*_{1s}$ (antibonding, empty). Two second-row atoms bring 8 valence atomic orbitals ( $2s$ and three $2p$ each), so their diagram has 8 valence MOs.