Read in this section
In diatomic species, there is only one important coordinate, defined as the internuclear axis. We discuss the molecular orbitals in terms of the overlap of the atomic orbitals along this axis. In polyatomic species, this description must be extended to allow for the fact that there are many internuclear axes.
The importance of the symmetry of a molecule in determining the form of the molecular orbitals can be investigated by considering the molecular orbitals of the H3 molecule.
In photoelectron spectroscopy, PES, the kinetic energies of electrons emitted from a sample when it is irradiated with high energy ultraviolet radiation are measured. The use of ultraviolet radiation is sometimes reflected in the name UV-PES.
The permanent magnetic moment of a molecule is due to the presence of unpaired electron spins in that molecule.
The electrons in an atom occupy the atomic orbitals from the lowest energy upwards, according to the aufbau principle, and Hund’s rule.
The Valence Shell Electron Pair Repulsion Theory is a way of predicting the shape of a molecule based on the number of bonding and lone pairs of electrons in a polyatomic species.
The structure of atoms is discussed in terms of the occupation of atomic orbitals by the electrons in that atom. Similarly, the structure of a molecule can be discussed in terms of molecular orbitals.
In the molecular orbital theory of H2, we consider the molecular orbitals as made up of the symmetric and antisymmetric combination of the individual 1s atomic orbitals on the two atoms. In general, however, there is more than one occupied orbital in the original atoms. The choice of the atomic orbitals needed to describe the molecular orbitals is known as the minimal basis set.
The Lewis scheme of bonding involves the formation of a covalent bond by the sharing of two electrons between two species. Double and triple covalent bonds have two and three sets of shared electron pairs, whilst lone pairs are valence electron pairs which do not contribute to bonding.