Before examining some of the important basic concepts of modern nuclear science, let us briefly review how the contemporary model of the nuclear atom emerged. Democritus (c. 460–370 B.C.E.) generally receives credit as being the first person to promote the idea of the atom—the smallest piece of an element indivisible by chemical means. Long before the emergence of the scientific method, this early Greek philosopher reasoned that if you continually divide a chunk of matter into progressively smaller pieces, you eventually reach the point beyond which subdivision is no longer possible. At that point, only indivisible little building blocks of matter, or “atoms,” remain. The modern word atom comes from the ancient Greek word ατοµος, meaning “indivisible.”
Despite Democritus’s clever insight, the notion of the atom as the tiniest, indivisible piece of recognizable matter languished in the backwater of human thought for more than two millennia. One reason for this intellectual neglect was that the great Greek philosopher Aristotle did not like the idea. Aristotle’s teachings dominated Western thinking for centuries. Another reason was that the precision instruments and machines needed to study nuclear phenomena effectively were only developed in the early part of the twentieth century. An exciting synergism occurred between the discovery of previously unimaginable nuclear phenomena and the emergence of new theories (concerning the nature of matter and energy) and more sophisticated equipment to validate these new theories experimentally.
Today, nuclear scientists perform both theoretical and experimental investigations of the processes that take place not only within the atomic nucleus but also deep within the very nucleons (i.e., the protons and neutrons) that make up the nuclei of all atoms, save ordinary hydrogen. An ordinary hydrogen atom has a single proton for its nucleus and a single electron in orbit around that nucleus. Despite its simple composition, the application of quantum theory to the hydrogen atom by the Danish scientist Niels Bohr (1885–1962) started quantum mechanics and stimulated a revolution in the theory of atomic structure.
In the early part of the nineteenth century, while attempting to explain observed mass differences and reactions between known chemical elements the English chemist John Dalton (1766–1844) revived atomic theory. Dalton constructed his atomic theory around the postulate of infinitesimally small, indivisible spheres of matter. Despite early resistance, the concept of the atom slowly gained acceptance within the scientific community. The process sped up a bit in 1869 when Dmitri I. Mendeleyev (1834–1907) published his famous periodic table, in which the Russian chemist assigned all the known chemical elements to specific groups.
Throughout the remainder of the nineteenth century, scientists generally remained comfortable with the basic assumption that atoms of a chemical element were simply very tiny, indivisible spheres. Contributing to this limited perception is the fact that all atoms, regardless of their atomic number (and, therefore, their chemical identity), have roughly the same physical size—a radius of about 10−10 meter. Furthermore, within the measurement limits of the day, the size of molecules and microscopic chunks of solid matter appeared to correspond well to the sum of the aggregate atomic sizes. Scientists including the Austrian theoretical physicist Ludwig Boltzmann (1844–1906) used the solid-atom model to develop important new scientific concepts, such as the kinetic theory of gases and the principles of statistical thermodynamics. However, few, if any, of these scientists dared to speculate that the tiny solid atom might actually have an internal structure—one characterized by even smaller, subatomic-sized particles and a vast quantity of empty space.
Sakharov's Noble Peace Prize
