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All the atoms of a given element have the same number of protons and electrons. The number of neutrons, however, may vary for atoms of the same element. For example, almost 99 percent of carbon atoms have six neutrons, but the rest have either seven or eight neutrons. Atoms of an element that differ in their numbers of neutrons are called isotopes. The nuclei of these isotopes of carbon are shown in the Figure below . The isotope called carbon-14 is used to find the ages of fossils. You can learn how at this URL: http://science. howstuffworks. com/environmental/earth/geology/carbon-14. htm .