Radioactive decay
Most carbon atoms have six protons and six neutrons in their nucleus.  This carbon is called carbon-12s (six protons + six neutrons = 12).  Its atomic weight is 12.  If a carbon atom has two more neutrons it is carbon-14. Carbon-14 acts chemically like other carbon, because the six protons and six electrons are what govern its chemical properties.  In fact, carbon-14 exists in all living things; all plants and animals contain carbon-14.  However, carbon-14 is radioactive. It decays by beta decay to become nitrogen-14. Carbon-14, in the small amounts found about us in nature, is harmless.  In archeology, this kind of carbon is used to determine the age of wood and other formerly living things. The method is called radiocarbon dating.