Isotope One of two or more atoms having the same Atomic Number but differing in Atomic Weight and Mass Number. The nuclei of isotopes of the same element have the same number of Proton (equal to the element's atomic number) but have different numbers of Neutrons. The isotopes of a given element have identical chemical properties but slightly different physical properties. A radioactive isotope, or radioisotope, is a natural or artificially created isotope having an unstable nucleus that decays emitting alpha, beta, or gamma rays (see Radioactivity) until stability is reached. For most elements, stable and radioactive isotopes are known.