Molecular orbital theory describes the orbitals of a molecule as bonding and antibonding combinations of atomic orbitals. Atomic orbitals (AO) combine to form molecular orbitals (MO) associated with the molecule rather than individual atoms. A molecular orbital is σ if the orbital lies directly along the internuclear axis, or π if the orbital does not lie along the internuclear axis. This theory explains bond formation and the magnetic and spectral properties of molecules