Claim: Carbon is an element.

Carbon -LRB- from carbo `` coal '' -RRB- is a chemical element with symbol C and atomic number 6 . It is nonmetallic and tetravalent -- making four electrons available to form covalent chemical bonds . Three isotopes occur naturally , C and C being stable , while C is a radioactive isotope , decaying with a half-life of about 5,730 years . Carbon is one of the few elements known since antiquity .   Carbon is the 15th most abundant element in the Earth 's crust , and the fourth most abundant element in the universe by mass after hydrogen , helium , and oxygen . Carbon 's abundance , its unique diversity of organic compounds , and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables this element to serve as a common element of all known life . It is the second most abundant element in the human body by mass -LRB- about 18.5 % -RRB- after oxygen .   The atoms of carbon can bond together in different ways , termed allotropes of carbon . The best known are graphite , diamond , and amorphous carbon . The physical properties of carbon vary widely with the allotropic form . For example , graphite is opaque and black while diamond is highly transparent . Graphite is soft enough to form a streak on paper -LRB- hence its name , from the Greek verb '' γράφειν '' which means `` to write '' -RRB- , while diamond is the hardest naturally occurring material known . Graphite is a good electrical conductor while diamond has a low electrical conductivity . Under normal conditions , diamond , carbon nanotubes , and graphene have the highest thermal conductivities of all known materials . All carbon allotropes are solids under normal conditions , with graphite being the most thermodynamically stable form . They are chemically resistant and require high temperature to react even with oxygen .   The most common oxidation state of carbon in inorganic compounds is +4 , while +2 is found in carbon monoxide and transition metal carbonyl complexes . The largest sources of inorganic carbon are limestones , dolomites and carbon dioxide , but significant quantities occur in organic deposits of coal , peat , oil , and methane clathrates . Carbon forms a vast number of compounds , more than any other element , with almost ten million compounds described to date , and yet that number is but a fraction of the number of theoretically possible compounds under standard conditions . For this reason , carbon has often been referred to as the `` king of the elements '' .