CHEM 121 glossary

Linear combination of atomic orbitals (LCAO)

The method of building molecular orbitals by adding and subtracting atomic orbitals on neighbouring atoms. The in-phase (added) combination piles up electron density between the nuclei and gives a bonding MO; the out-of-phase (subtracted) combination has a node between the nuclei and gives an antibonding MO.

Head-on overlap of two atomic orbitals; adding them in phase gives a σ bonding MO.

Example

Two H $1s$ orbitals: $1s_A + 1s_B$ gives $\sigma_{1s}$ , and $1s_A - 1s_B$ gives $\sigma^*_{1s}$ . Two orbitals go in and two MOs come out; orbitals combine strongly only when they are close in energy and overlap well.