Radioisotope
An isotope of an element whose nucleus is unstable and undergoes radioactive decay.
Unit
Dimensionless
In depth
Radioisotopes behave chemically just like the stable isotopes of the same element, which is what makes them such useful tracers: they follow the identical chemical pathway while announcing their position by their radiation. They may occur naturally or be made in reactors and accelerators.
Examples in the real world
Carbon-14 for dating, iodine-131 for thyroid treatment, cobalt-60 for sterilising equipment.