Carbon

It is nonmetallic and tetravalent — meaning that four electrons are available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three isotopes occur naturally: ¹²C and ¹³C are stable, while ¹⁴C is a radionuclide, 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 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 allow 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 (about 18.5%), after oxygen.

Carbon atoms can bond together in various ways, resulting in different allotropes of carbon. Well-known allotropes include graphite, diamond, amorphous carbon and fullerenes. The physical properties of carbon vary widely depending on the allotropic form. For example, graphite is opaque and black, whereas diamond is highly transparent. Graphite is soft enough to leave a streak on paper (hence its name, from the Greek verb "γράφειν", meaning "to write"), while diamond is the hardest naturally occurring material known. Graphite is a good electrical conductor, whereas diamond has 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 at standard temperature and pressure. They are chemically resistant and require high temperatures 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 in 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 an enormous number of compounds, more than any other element, with almost ten million compounds described to date — and yet this figure is only 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".

The allotropes of carbon include graphite, one of the softest known substances, and diamond, the hardest naturally occurring substance. It bonds readily with other small atoms, including other carbon atoms, and is capable of forming multiple stable covalent bonds with suitable multivalent atoms. Carbon is known to form almost ten million compounds, the vast majority of all chemical compounds. Carbon also has the highest sublimation point of all the elements. At atmospheric pressure it has no melting point, as its triple point lies at 10.8 ± 0.2 megapascals (106.6 ± 2.0 atm; 1,566 ± 29 psi) and 4,600 ± 300 K (4,330 ± 300 °C; 7,820 ± 540 °F), so it sublimes at approximately 3,900 K (3,630 °C; 6,560 °F). Graphite is much more reactive than diamond under standard conditions, despite being thermodynamically more stable, because its delocalised pi system is far more vulnerable to attack. For example, graphite can be oxidised by hot concentrated nitric acid under standard conditions to mellitic acid, C₆(CO₂H)₆, which preserves the hexagonal units of graphite while breaking up the larger structure.

Carbon sublimes in a carbon arc, which has a temperature of about 5,800 K (5,530 °C or 9,980 °F). Thus, regardless of its allotropic form, carbon remains solid at higher temperatures than the highest-melting metals such as tungsten or rhenium. Although carbon is thermodynamically susceptible to oxidation, it resists oxidation more effectively than elements such as iron and copper, which are weaker reducing agents at room temperature.

Carbon is the sixth element, with a ground-state electron configuration of 1s² 2s² 2p², of which the four outer electrons are valence electrons. Its first four ionisation energies — 1086.5, 2352.6, 4620.5 and 6222.7 kJ/mol — are much higher than those of the heavier group 14 elements. The electronegativity of carbon is 2.5, significantly higher than that of the heavier group 14 elements (1.8–1.9), but close to that of most nearby nonmetals as well as some second- and third-row transition metals. The covalent radii of carbon are normally taken as 77.2 pm (C–C), 66.7 pm (C=C) and 60.3 pm (C≡C), although these may vary depending on the coordination number and on what the carbon is bonded to. In general, the covalent radius decreases with lower coordination number and higher bond order.

Carbon-based compounds form the basis of all known life on Earth, and the carbon–nitrogen cycle provides part of the energy produced by the Sun and other stars. Although it forms an extraordinary diversity of compounds, most forms of carbon are relatively unreactive under normal conditions. At standard temperature and pressure it resists all but the strongest oxidants. It does not react with sulfuric acid, hydrochloric acid, chlorine or any bases.