Carbon isotopes come in three forms, the most common isotope of carbon is carbon-12 (12C), which contains six neutrons in addition to its six protons. The next heaviest carbon isotope, carbon-13 (13C), has seven neutrons. Both 12C and 13C are called stable carbon isotopes since they do not decay into other forms or elements over time. The rare carbon-14 (14C) isotope contains eight neutrons in its nucleus. Unlike 12C and 13C, this isotope is unstable, and radioactive. Over time, a 14C atom will decay into a stable product. Stable carbon isotopes are considered to be one of the most important isotopes in petroleum geochemistry as the combination of 12C and 13C can often be used to identify oil sources, genetic origin of natural gases and hydrocarbon thermal maturity.