ABSTRACT
A clear knowledge of the structure of atoms has been acquired since the first decade of the present century. Experiments by Rutherford implied that the atom was composed of a positive nucleus and orbiting electrons. A theory by Niels Bohr explained the electronic behavior. Later developments in quantum theory replaced notions of fixed electron orbits by a more insubstantial concept and the advent of wave mechanics enabled the interpretation of atomic phenomena of greater complexity. In this chapter we consider only the consequences of theoretical approaches to atomic and subatomic structures.
The “solar system” model is still used to visualize the atom. In this representation, light negatively charged electrons “orbit” around a heavy, positive nucleus. The nucleus contains two kinds of nucleons, one of which is positively charged (the proton) and the other neutral (the neutron). However, experiments show that, in addition to electrons and nucleons, the subatomic world consists of hundreds of constituents, some of them being composite entities. Particle physics suggests that this multitude can be reduced to only two types of elementary particles: quarks and leptons.
Four fundamental forces operate in nature and are classified as electromagnetic force, strong force, weak force, and gravitational force. They are thought to operate by way of force-carrier particles called gauge bosons.
The gauge bosons, quarks, and leptons are believed to be the truly elementary particles, i.e., the constituents of matter which do not consist of anything smaller.
About 300 kinds of nuclei are stable. Hundreds of other natural or artificial ones are ephemerous because of radioactivity. Radioactive decay is a spontaneous disintegration of the nuclei with emission of energy in the form of particles or electromagnetic radiation. It is a statistical phenomenon and the time required for half of a given number of atoms to be transformed is called the half life. It may range from millions of years to a fraction of a second.
Presently existing radioactive elements and the remnants of decayed radionuclides are used as “radioactive clocks” in dating various objects and events not only on a historical time scale, but also within the solar system and in outer space.
