 The three main isotopes of carbon are carbon-12, carbon-13, and carbon-14. Carbon-12 has six protons and six neutrons. Carbon-13 has six protons and seven neutrons. And carbon-14 has six protons and eight neutrons. Carbon-14 is a radioactive isotope and disappears after 100,000 years. In addition, carbon-12 is much more abundant in nature than carbon-13. Because carbon-12 and carbon-13 have different atomic weights, these isotopes are fractionated via a number of different biological processes. The main process that fractionates carbon-12 in nature is photosynthesis. Because carbon-12 is much lighter than carbon-13, the organic material formed via photosynthesis is enriched in carbon-12. And the material from which this organic material forms, the atmosphere or the ocean, remains enriched in the other isotope, carbon-13. Other processes that fractionate carbon-12 and carbon-13 include respiration and the formation of methane. Because biological processes operate over time and fractionate carbon-12 and carbon-13, the ratios of carbon-12 and carbon-13 in natural materials has changed significantly over time. And this fractionation has a number of different applications, as we will see later in the module.