CHEM 121 glossary

MO diagram

An energy-level diagram of a molecule: the atomic orbitals of each atom on the two sides and the molecular orbitals they form in the middle, ordered by energy and filled with the valence electrons (lowest first, following Pauli and Hund). From the filled diagram you read the bond order, the number of unpaired electrons and the HOMO.

\ce{N}

atom

\ce{N2}

MOs

\ce{N}

atom

$\ce{N2}$ : s–p mixing puts $\pi_{2p}$ below $\sigma_{2p}$ .

Example

$\ce{N2}$ (10 valence electrons, s–p mixed order): $(\sigma_{2s})^2(\sigma^*_{2s})^2(\pi_{2p})^4(\sigma_{2p})^2$ . That is 8 bonding and 2 antibonding electrons, bond order 3, all paired (diamagnetic), with $\sigma_{2p}$ as the HOMO.