The process of giving off atomic particles by radioactive atoms is called radioactive decay. In radioactive decay one element is changed into another. Its rate depends on the form of an element and on the element. A group of radioactive elements formed by the decay of one element into another is called a radioactive series. The rate at which elements decay is expressed in terms of half-lives.
Radioactivity produces transmutations in elements, that is, it changes one element into another. In the Periodic Table, elements are classified by their atomic numbers. When an atom emits an alpha or beta particle, the atom becomes a different element, thereby changing its place in the Periodic Table.
An atom that emits an alpha particle loses two protons and two neutrons. The loss of the two protons reduces its atomic number by two. This change is shown by the diagram Radioactive Decay. For example, the first element in the diagram is uranium, which has an atomic number of 92. When it emits an alpha particle, it becomes thorium, which has an atomic number of 90.
There are three main types of beta decay. In the most common type, a neutron in the nucleus of an atom changes into a proton. A negative beta particle (an electron) and a particle called an antineutrino are formed in the process and are emitted from the nucleus. This type of beta decay results in the atomic number of the nucleus increasing by one. Each case of beta decay shown in the diagram is of this type. For example, thorium (atomic number 90) becomes protactinium (atomic number 91). In the other two types of beta decay, the atomic number decreases by one. In both, a proton changes into a neutron and a particle called a neutrino is formed and emitted from the nucleus. In one, a positive beta particle (a positron) is also formed and emitted from the nucleus; in the other, an inner electron of the atom is captured by the nucleus and destroyed.
The weight of both neutrons and protons is roughly equal to one unit of atomic weight. In comparison, the weight of electrons, positrons, neutrinos, and antineutrinos is negligible. Thus when an atom emits an alpha particle, its atomic weight decreases by four units, but when an atom emits a beta particle, its atomic weight remains the same. For example, when thorium 230 decays, it emits an alpha particle and becomes radium 226. On the other hand, when lead 214 decays, its atomic weight remains at 214, although it becomes an atom of a different element (bismuth).
Isotopes are atoms with the same atomic number but differing atomic weights because they have unequal numbers of neutrons. Isotopes of the same element have identical chemical properties but slightly different physical ones. Many of the isotopes of naturally occurring elements are radioactive. The decay properties of radioactive isotopes of the same element usually differ greatly.
As certain radioactive isotopes decay they produce other isotopes of other elements that are also radioactive. These daughter isotopes produce still other radioactive daughters, and so on until a stable isotope of some element is formed. Radioactive isotopes of this kind form radioactive series.
Three radioactive series occur in nature: the thorium series, the uranium series, and the actinium series. (The diagram Radioactive Decay shows the uranium series.) The thorium series begins with thorium 232, the uranium series with uranium 238, and the actinium series with uranium 235. Each series ends in a stable lead isotope. The neptunium series is produced artificially in nuclear reactors. It begins with neptunium 237 and ends with bismuth 209.
Every radioactive isotope decays at its own fixed, unchangeable rate. As time passes, more and more of the isotope changes into an isotope of another element. Eventually half the parent isotope changes. In an equal length of time, half of what now remains of the parent isotope will change, and so on. The time it takes for half of a radioactive isotope to change into another isotope is called the half-life of the parent isotope.
Uranium 238 has a half-life of 4.5 billion years. This means that in 4.5 billion years, half of a given amount of uranium 238 will have changed to thorium 234. In 24.1 days, half of a sample of thorium 234 will have become protactinium 234. The half-lives of other radioactive substances range from a very small fraction of a second to more than 100,000,000,000 years.