Isotopes have been found to have the same atomic number, and because of this, nearly identical chemical behavior, but a different atomic mass.  Most elements found in nature are mixtures of several isotopes.  Tin, for example, has 10 isotopes.  In most cases only stable isotopes of elements are found in nature.  The radioactive forms break down spontaneously into different elements.  Isotopes of all elements heavier than bismuth are radioactive, some occur naturally because they have long half-lifes.