Some Elements of Atomic Structure Theory

As everyone knows, an atom is the smallest unit of a chemical element, and atoms combine to form molecules and solids. Each element is uniquely specified by its atomic number Z. However a given element may occur in several different versions, called isotopes; these have the same Z but different atomic mass number A. A glance at the periodic table shows that for the naturally occurring elements Z and A vary from hydrogen (Z = 1, A = 1) to uranium (Z = 92, A = 238). The mass of an atom is given approximately by A, in terms of the atomic mass unit, or amu: 1 amu = 1.66 × 10−27 kg. The sizes of atoms are conveniently measured in terms of a unit called the Bohr radius, a0, which has the value 1 a0 = 5.3×10−11 m = 0.053 nm. Although atoms vary greatly in mass and other properties, their sizes are all comparable, being a few times a0. Since this size is small compared to the wavelength of light, atoms are not “visible” in the ordinary sense of the word – an optical microscope cannot form an image of a single atom. This is one of the reasons that the structure of an atom cannot be understood in the same familiar mechanical-geometrical terms that we use to describe the structure of a mountain, a tree, a crystal, or even a molecule. In many ways atomic physics is the prototype for the scientific study of objects which can be understood only by rather formal abstract ideas.