Most carbon atoms have six neutrons. Although carbon is defined as having six protons, the number of neutrons in a carbon nucleus can vary, which gives rise to the various isotopes of carbon. Fifteen isotopes of carbon, ranging from two to 16 neutrons, have been observed by scientists.
Carbon-12, or carbon with six neutrons, is stable, and makes up about 99 percent of all carbon.The remaining 1 percent is mostly Carbon-13, the other stable isotope of carbon, which has seven neutrons. Carbon-14, with eight neutrons, occurs naturally in trace amounts and is often used to determine the age of very old objects, because carbon-14 decays at a slow, but consistent rate.