Electronic Structure of Atoms

In this Chapter the electronic structure of single atoms will be discussed. A few quantum concepts will be introduced, as they are necessary for the understanding of many aspects in solid state physics and device applications.

In Chapter 1, we saw that matter was composed of atoms in the periodic table shown in Fig. 1.2. Until 1911, atoms were considered the simplest constituents of matter. In 1911, it was discovered that atoms had a structure of their own and Rutherford proposed the nuclear model of the atom in which almost all the mass of the atom is concentrated in a positively charged nucleus and a number of negatively charged electrons are spread around the nucleus. It was later found that the nucleus is itself made up of protons (positively charged) and neutrons (neutrally charged). The number of protons is the atomic number (Z) while the total number of protons and neutrons is the mass number of the element. Apart from the electrostatic repulsion between nuclei, all of the major interactions between atoms in normal chemical reactions (or in the structures of elemental and compound substances) involve electrons. It is therefore necessary to understand the electronic structure of atoms. The term electronic structure, (or configuration) when used with respect to an atom, refers to the number and the distribution of electrons about the central nucleus.

The following discussion traces the steps of the scientific community toward a description of the electronic structure of atoms. The reader should not be stopped by the new concepts that arise from this discussion, because they will become clearer after understanding the quantum mechanics presented in Chapter 3. Much of the experimental work on the electronic structure of atoms done prior to 1913 involved measuring the frequencies of electromagnetic radiation (e.g. light) that are absorbed or emitted by atoms. It was discovered that atoms absorbed or emitted only certain, sharply defined frequencies of electromagnetic radiation. These frequencies were also found to be characteristic of each particular element in the periodic table. And the absorption or emission spectra, i.e. the ensemble of frequencies, were more complex for heavier elements. Before being able to understand the electronic structures of atoms, it was natural to start studying the simplest atom of all: the hydrogen atom, which consists of one proton and one electron.