CHEM 121 glossary

Antibonding molecular orbital

A molecular orbital from the out-of-phase combination of atomic orbitals, with a node between the nuclei; it is marked with a star ( $\sigma^*$ , $\pi^*$ ). It lies higher in energy than the atomic orbitals, so each electron in it cancels a bonding electron and lowers the bond order by ½.

\ce{He}

atom

\ce{He2}

MOs

\ce{He}

atom

$\ce{He2}$ : the filled $\sigma^*_{1s}$ cancels the filled $\sigma_{1s}$ .

Example

$\ce{He2}$ would put 2 electrons in $\sigma_{1s}$ and 2 in $\sigma^*_{1s}$ : bond order ½(2 − 2) = 0, so helium stays monatomic. With one antibonding electron fewer, $\ce{He2+}$ (bond order ½) does exist.