Most archaeologists are familiar with the term isotope from their knowledge of radiocarbon dating. Isotopes are simply atoms of the same chemical element which have slightly different masses. Lead, for example, has four stable isotopes, designated by the symbols Pb204, Pb206, Pb207, and Pb208, which represent atoms weighing, respectively, 204, 206, 207 and 208 atomic mass units. For the great majority of chemical elements, the relative proportions of their isotopes are the same regardless of where the elements are found in the earth's crust. For instance, copper in natural minerals always contains the same proportions of Cu63 and Cu65; and silver always contains the same proportions of Ag'07 and Ago'09. Lead, however, is unusual, in that the relative proportions of its isotopes vary somewhat among ores occurring in different geographical areas. These variations reflect differences in the geological ages of the ore deposits and stem from the fact that Pb206, Pb207 and Pb208 are formed as end products of the radioactive decay series of uranium and thorium.