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).
[Illustration: Figure 8 _Isotopes of hydrogen._]
Hydrogen (¹H)
Deuterium (²H)
Tritium (³H)
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.