CHEM 121 glossary

s–p mixing

The interaction between the $\sigma_{2s}$ and $\sigma_{2p}$ MOs when the atoms' $2s$ and $2p$ energies are close, which pushes $\sigma_{2p}$ above the $\pi_{2p}$ pair. In this course it applies to diatomics of B, C and N (and $\ce{CO}$ , $\ce{CN-}$ ); for O, F and Ne the $2s$ – $2p$ gap is large and $\sigma_{2p}$ stays below $\pi_{2p}$ .

\ce{C}

atom

\ce{C2}

MOs

\ce{C}

atom

$\ce{C2}$ in the mixed order: both $\pi_{2p}$ MOs full, $\sigma_{2p}$ empty.

Example

$\ce{C2}$ has 8 valence electrons. In the mixed order the last 4 fill both $\pi_{2p}$ MOs, so $\ce{C2}$ is diamagnetic, as observed; the unmixed order would put 2 in $\sigma_{2p}$ and leave 2 unpaired in $\pi_{2p}$ . Either way the bond order is 2.