Isotopes

Atoms of the same element, that is, atoms with the same number of protons and electrons, may vary slightly in mass because of having different numbers of neutrons. Since the chemical behavior of an element depends upon its electrons’ electrical charges, extra neutrons (which do not have an electrical charge) may affect the mass of an atom without disturbing its chemical properties. Atoms having the same atomic number but different atomic weights are called isotopes. For example, as shown in Figure 8, the isotope ¹H, or ordinary hydrogen, consists of a nucleus containing a proton (charge: +1; mass: 1) around which revolves an electron (charge: -1; mass: negligible); ²H, known as deuterium, contains an additional nuclear particle, a neutron (charge: 0; mass: 1); ³H, or tritium, contains two neutrons. Since the chemical behavior of an element depends upon the number of its electrons, these three atoms, although differing in weight, behave identically in chemical reactions. For convenience, the atomic weight is written as a superscript to the left of the element’s symbol. For instance ¹⁴C is the isotope of carbon with an atomic weight of 14 (ordinary carbon is the isotope with an atomic weight of 12, and it is written ¹²C).

Practically all elements have more than one isotope. There are two general classes of isotopes, stable and radioactive. Stable isotopes have no distinguishing characteristic other than their mass; radioactive isotopes not only differ from their brothers in mass but also are characterized by unstable nuclei. When the nucleus of an atom is unstable, because of an unbalanced number of protons and neutrons, a redistribution occurs sooner or later, and the atom decomposes spontaneously and emits one of several kinds of radiations. Because of their common mode of action and effects on living organisms, these different kinds of radiations are known collectively as ionizing radiations.

All radioactive elements emit one or more of three types of penetrating (ionizing) rays. Alpha rays or particles are double-charged helium nuclei, ⁴He (atomic number: 2; mass: 4). They are emitted by many heavy radioactive elements, such as radium, uranium, and plutonium. Beta rays or particles can be either positive or negative. Negative beta particles are high-speed electrons and are emitted by many radioactive elements. Positive beta particles are positively charged electrons (positrons), have only a transitory existence, and are less common. Gamma rays are electromagnetic radiations, a term that also describes radiowaves, infrared rays, visible light, ultraviolet light, and X rays. Gamma rays are usually emitted after the emission of alpha or beta particles. In our studies of life processes, we are interested only in the radioactive isotopes that emit gamma rays or beta particles.