What is the difference between a radioactive isotope and a stable isotope?

Chemically, they are the same; however, the radioactive isotope will eventually emit a particle and/or radiation, and in doing so may change into an atom of another element! For instance, potium-40 may change into argon-40. This fact enables geologists to date igneous rock formations, inasmuch as the argon cannot escape the crystal lattice of the mineral until the rock is treated for ysis.