Carbon constitutes the sixth-most-abundant element in the universe, accounts for 20 percent of the weight of almost all living organisms, and makes up roughly 0.04 percent of the Earth's atmosphere in the form of carbon dioxide. Scientists also measure amounts of carbon-14 in materials for the purposes of carbon dating.
Carbon, referred to on the periodic table of elements by the symbol C, forms within stars through a reaction referred to as the triple-alpha process. Older stars with depleted levels of hydrogen accumulate large amounts of helium that eventually fuse under very high temperatures. The ensuing chain reaction produces carbon as an end result. Six protons, six electrons, and seven neutrons make up the atomic structure of carbon, giving carbon an atomic weight of 12.0107 and the atomic number 6. Its name has its origins in the Latin word "carbo," which means coal.
Widely used forms of carbon include charcoal, diamond, graphene and early forms of tattoo ink. As of 2015, scientists estimate that carbon serves as the keystone for 95 percent of known compounds and have discovered nearly 10 million different carbon compounds. Carbon in the form of coal also serves a pivotal role in the steel production process while many industrial lubricants utilize graphite, another form of carbon.