Nuclear chemistry is the field of chemistry that deals with transformations in an atomic nucleus. In most cases, the atomic nuclei that occur in nature are stable; that is, they do not spontaneously change into some other form. For example, the nucleus of the most common kind of carbon atom that occurs in nature (carbon-12) consists of six protons and six neutrons. One could wait thousands or millions of years, and that carbon nucleus would still have the same composition. In fact, there is no reason to believe that it would ever change into some other form, with different numbers of protons and neutrons. Some of the nuclei that occur in nature, however, are not stable. As an example, the nucleus of an isotope of carbon-12, carbon-14, contains six protons and eight neutrons. If one waits around for a period of time, the carbon-14 nucleus breaks apart. It gives off an electron (formed by the breakdown of a neutron in the nucleus into a proton and an electron), leaving behind seven protons and seven neutrons. The carbon-14 nucleus has been transformed in the process into a nitrogen-14 nucleus. One way to describe that change is to write a nuclear equation like the following:
146C → -10e + 147N + νe.
In this (and all other nuclear equations) the superscript number presents the mass number of the nucleus (the total number of protons and neutrons) and the subscript represents the atomic number of the nucleus (the number of protons). The letter "e" represents an electron, called a beta particle when it is emitted from a nucleus, and the Greek letter ν stands for a particle called an anti-neutrino, which carries away some of the energy produced in the reaction.
The process by which an atomic nucleus spontaneously emits a subatomic particles, such as an electron, is known as radioactivity or radioactive decay. Atomic nuclei can undergo radioactive decay in a number of ways, one of which is beta decay, as shown in the example of carbon-14 above. Another type of radioactive decay occurs when an atomic nucleus loses a small particle consisting of two protons and two neutrons, a combination called an alpha particle. An alpha particle has the same composition as the nucleus of a helium atom. So, it can be represented by the symbol 42He. A third type of radioactive decay is called gamma decay because it occurs when a nucleus emits a burst of energy known as a gamma ray. A gamma ray is a high-energy form of electromagnetic radiation. Gamma ray can occur by itself, although it most commonly occurs in conjunction with alpha or beta decay.
A number of other forms of radioactive decay are also possible, although most are less common than alpha, beta, or gamma decay. For example, a nucleus may, on rare occasions, emit a single neutron or a single proton. In other instances, a nucleus may capture an electron orbiting around it and use the electron to convert a proton into a neutron. There are even instances in which a nucleus decays with the emission of two beta rays simultaneously.
Changes can also be made to occur in an atomic nucleus artificially. When a small particle, such as a proton or neutron, is fired at an atom, there is some likelihood that that particle will be captured by the nucleus of the atom. When that occurs, the composition of the nucleus changes, and it becomes a new nucleus of a new isotope or new atom. A common example of this reaction is the so-called n,γ reaction, in which a nucleus absorbs a neutron and emits a gamma ray. For example, suppose that the nucleus of a carbon-12 atom is struck by a neutron, which it then absorbs. The n,γ reaction that occurs is as follows:
126C + 10n → -136C + 00γ. Changes made in an atomic nucleus artificially are sometimes known as induced nuclear transformations. They have a number of uses in nuclear chemistry. For example, the n,γ reaction is often used to change a stable isotope into a radioactive isotope of the same kind. One of the most useful tools in producing such reactions is a particle accelerator, which fires subatomic particles, such as electrons and protons, at atomic targets, causing a variety of changes in their nuclei.