Practically everyone nowadays is to some extent familiar with the atomic
structure of matter. Atomic energy, nuclear reactors, and radioisotopes
are terms in everyday usage. However, to appreciate how radioisotopes
can be applied to the study of life processes, we must have at least a
working knowledge of their properties, their preparation, and their
limitations. It is therefore appropriate to examine them in detail so
that the succeeding chapters will be more easily understood.
According to present-day theory, an atom consists of a nucleus[4] that
is made up of protons and neutrons[5] and is surrounded by electrons. In
each atom there is an equal number of protons (positively charged) in
the nucleus and electrons (negatively charged) moving concentrically
around the nucleus; since neutrons have no electrical charge and since
protons and electrons cancel each other’s charges, the whole atom is
electrically neutral, or uncharged. Each atom is identified by an atomic
number and an atomic weight. The atomic number of an element (for
example, carbon, nitrogen, oxygen) is determined by the number of
protons, or positive charges, carried by the nucleus (or by the number
of electrons surrounding the nucleus, which is the same). The atomic
weight is the weight of an atom as compared with that of the atom of
carbon, which is taken as a standard. The weight, or mass, of an atom is
due chiefly to its protons and neutrons because the mass of its
electrons is negligible.