Different forms of the same element are called isotopes. Isotopes have the same number of positive particles, called protons, and negative particles, called electrons, but differing numbers of neutrons. Neutrons are neutral particles residing in the nucleus of an atom along with protons.
All elements on the periodic table have isotopes, although some are artificially created. Heavier elements tend to have more isotopes than lighter elements.
Although the characteristics of isotopes of an element are generally the same, their masses differ. The mass of an atom consists of the number of protons and neutrons. Some isotopes of an element are heavier or lighter than others, depending on how many more or fewer neutrons the isotope has.
Some isotopes are stable, and others are unstable. Isotopes that are unstable break down, or decay, into other elements, giving off radiation in the process. This decay process allows the atom to become more stable.