Carbon (from Latin carbo 'coal') is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes up about 0. 025 percent of Earth's crust. Three isotopes occur naturally, 12C and 13C being 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 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 (about 18. 5%) after oxygen. The atoms of carbon can bond together in diverse ways, resulting in various allotropes of carbon. Well-known allotropes include graphite, diamond, amorphous carbon, and fullerenes. 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 (hence its name, from the Greek verb "γράφειν" which means "to write"), 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 at standard temperature and pressure. 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, with about two hundred million having been described and indexed; and yet that number is but a fraction of the number of theoretically possible compounds under standard conditions.

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Atomic properties

Standard atomic weight12.011 ±0.002 [12.0096 … 12.0116]
Atomic mass12.0111 u

Atomic radii

Radius (empirical)70 pm
Radius (calculated)67 pm
Covalent radius76 pm
Van der Waals radius170 pm

Atomic shell

Electron configurationHe 2s2 2p2
Ionization energy(1st) 11.260288 eV
(2nd) 24.383154 eV
(3rd) 47.88778 eV
(4th) 64.49352 eV
(5th) 392.09051 eV
Shell model

Physical properties

Densityamorphous: 1.95 [1.8 … 2.1] g·cm−3
Graphite: 2.26 g·cm−3
Diamond: 3.51 g·cm−1
Molar volume5.31·10-6 m3·mol−1
Speed of soundDiamond: 18,350 m·s−1 (293.1 K)


Sublimation point3,915 K
Triple point4,600 K @ 10.8 MPa
Transition temperature15 K


Melting enthalpy117 kJ·mol-1
Enthalpy of vaporization715 kJ·mol-1
Binding energy717 kJ·mol-1

Heat and conductivity

Specific heat capacityGraphite: 427 J·kg−1·K−1
Diamond: 709 J·kg−1·K−1
Thermal conductivity140 W·m-1·K-1
Expansion coefficient7.1·10-6 K-1
Work function4.81 eV


Mohs hardness0.51

Elastic properties

Bulk modulus33 GPa

Electrical properties

Resistance1·10-5 Ωm


Magnetic susceptibility-5.9·10-6 cm3·mol−1

Optical properties

Reflectivity27 %
Refractive indexDiamond: 2.417

Chemical properties

Basicityslightly acidic
Oxidation state-4 … 4


Pauling scale2.55
Sanderson scale2.75
Allred-Rochow scale2.5
Mulliken scale2.48
Allen scale2.544
Ghosh-Gupta scale6.233 eV
Boyd-Edgecombe scale2.6
Nagle scale2.55
Pearson absolute negativity6.27 eV

Other properties

Natural occurrenceprimordial
Crystal structureHexagonal
Goldschmidt Classificationatmophile
Superconductorwith transition tempperature under special conditions
Price/kg0.122 USD

Natural abundances

5·106 ppb ≈ 5.02·1016 M☉
3·106 ppb ≈ 5.96·1015 Mt
1.5·107 ppb ≈ 15 kg
Earth’s crust
1.8·106 ppb ≈ 4.98·107 Mt
28,000 ppb ≈ 38.3 Mt
Flowing water
1,200 ppb ≈ 19.2 Gt
Human body
2.3·108 ppb ≈ 16.1 kg