Molecular orbitals and structure

This chapter examines molecular orbitals, which are single-electron wavefunctions that describe the behaviour of electrons in molecules. Molecular orbital (MO) theory relies on two important approximations: the orbital approximation and the linear combination of atomic orbitals. The orbital approximation is the assumption that the overall wavefunction for an atom containing N-electrons can be approximated by the product of N single-electron atomic orbitals. Extending this idea to molecules leads to the assumption that the overall wavefunction for an N electron molecule can be approximated by the product of N single-electron molecular orbitals. Wavefunctions have wave characteristics and therefore, atomic orbitals on one atom can interfere with those on another atom. The interference can be constructive (addition) or destructive (subtraction). We can therefore use an approach called the linear combination of atomic orbitals (LCAO) to build up molecular orbitals via addition and subtraction of atomic orbitals.