Transmutation, the changing of one chemical element into another. Transmutation involves a change in the nucleus, or core, of an atom and is, therefore, a nuclear reaction. An atom contains a nucleus (made up of protons and neutrons) surrounded by electrons. The number of protons gives the atom its identity as a chemical element. When the number of protons in an atom is changed, the atom is transmuted into an atom of another element. Transmutation may be either natural or artificial.

Natural, or spontaneous, transmutation occurs in unstable, radioactive elements, which decay into a series of other elements until a stable element is formed. Thus, uranium 238 spontaneously transmutes, through a series of steps, into lead 206, which is stable. For information on the types of spontaneous transmutation,

Artificial, or induced, transmutation occurs when atoms of one element are bombarded with particles in a nuclear reactor or a particle accelerator and are altered. All of the transuranium elements (elements with atomic numbers greater than 92, such as plutonium and americium) are man-made through induced transmutation. Most nuclear reactions involve artificially transmuting elements, although such reactions are usually referred to by terms denoting the specific nature of the process (fission, fusion, irradiation) or the desired end-product (radioisotope production) rather than by the blanket term transmutation.

There are many different methods of producing artificial transmutation. Particle accelerators may be used to bombard elements with alpha particles (two protons and two neutrons), deuterons (one proton and one neutron), or nuclei of atoms of elements such as carbon or boron. In these cases, some or all of the protons in the bombarding particles may become lodged in the target nucleus, transmuting it into a different element. In a nuclear reactor, the target nucleus is bombarded with neutrons, causing the nuclei to fission (split) or merely to become radioactive. (The radioactive nuclei then transmute spontaneously.)

In the Middle Ages, alchemists tried to transmute base metals into gold. They failed because their methods were chemical and did not alter atomic nuclei. In 1919 Ernest Rutherford of Great Britain produced the first artificial transmutation when he bombarded the nucleus of a nitrogen isotope with alpha particles and produced an isotope of oxygen and a proton.