In order to understand qualitatively some of the radiative and chemical processes discussed in the text a general appreciation of the structure of the electron cloud around atoms is necessary. Table B.1 gives the Periodic Table of the naturally occurring elements and Table B.2 shows the configuration of the electrons in the orbitals surrounding the appropriate atomic nucleus. The reader will probably be aware of the similarity of chemical properties exhibited by the elements in vertical columns of the Periodic Table. Table B.2 suggests that this has much to do with the electronic structure.

In Chapter 1 we encountered the concept of energy within atoms or molecules being quantized, or only allowed a discrete number of values. The theory of quantum mechanics extends the ideas of classical mechanics to such systems. In 1926 Schrödinger developed an equation to describe the position and energy of the electron around the hydrogen atom, using the concept that the total energy is the sum of kinetic and potential energies. The solution to this equation allows only discrete energies, and electron orbital shapes, to exist. The orbitals are not the exact tracks which the electron may follow but give the region of space around the nucleus where the electron is most likely to be found when it possesses a particular energy.