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. Transformations may result from nuclear decay of radioactive isotopes, or they can result from neutron capture. Neither necessarily involves a change in the element of the atom.
When the number of protons in an atom is changed, the atom is transmuted into an atom of another element.
A neutron capture can leave the atomic number the same, but always increases the number of nucleons. This changes the isotope number but the change is not chemical, since the atom is still what it had been.
Natural 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 into lead 206, which is stable.