Atomic number 6

Carbon

C
+ phase
− phase

Shells Orbitals 2s 2p

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

6 electrons in 2 shells: 2, 4.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Nonmetal
Period: 2
Group: 14
Block: p
Standard atomic weight: 12.011(2) Da
Electron configuration: [He] 2s² 2p²
Phase at 25 °C: solid
Density: 2.2670 g/cm³
Electronegativity (Pauling): 2.55
Discovered: Prehistoric times, as charcoal and soot

Neighbours in the table

B N

A carbon atom carries six electrons in two shells, two in the first and four in the second. Those four outer electrons have four orbitals to fill, one 2s and three 2p, each a region where an electron is likely to be found. Four orbitals means four bonds at most; the larger atoms below carbon in its group can take five or six. Its electronegativity, the pull an atom has on the electrons in a bond, is 2.55 on the Pauling scale. That is mid-range: carbon shares electrons rather than taking or giving them.

The atom is also small, so its orbitals overlap a neighbour’s closely. That lets carbon form pi bonds, the sideways overlap that turns a single bond into a double or triple one. Carbon atoms also link to one another, and carbon-carbon single bonds are very strong. Close to ten million of its compounds are known, from methane and acetylene to benzene.

In diamond each atom makes four single bonds to four neighbours at the corners of a tetrahedron, so the whole crystal is a single molecule. In graphite each atom bonds to three neighbours instead, in flat sheets of six-membered rings. The fourth electron is free to move through the sheet, which is why graphite conducts electricity and diamond does not.

Where it is found

Only five elements are more abundant in the universe than carbon, and it shows up in the sun, in stars, in comets and in the atmospheres of most planets. The atmosphere of Mars is 96.2% carbon dioxide. Some meteorites carry diamonds too small to see without a microscope.

On Earth the crust holds about 200 milligrams of carbon per kilogram and seawater about 28 milligrams per litre. In combination it is the carbon dioxide in the air, and every natural water has some in solution. Whole rock masses are calcium carbonate, with magnesium and iron carbonates alongside it. Coal, petroleum and natural gas carry their carbon as hydrocarbons instead, compounds of carbon and hydrogen alone.

Uncombined, it turns up as graphite and diamond, and as charcoal, coke, carbon black, graphene and fullerene. Commercial supply comes mostly from coal deposits, processed before it is fit for use. Natural diamonds are mined from kimberlite in ancient volcanic pipes. They are now also taken from the seabed off the Cape of Good Hope.

Why it matters

In 1961 IUPAC, the body that sets chemistry’s standards, made carbon-12 the basis of the atomic weight scale. Every other element’s mass is quoted against it: carbon-12 weighs exactly 12 Da by definition, and 12 g of it holds 6.02×10²³ atoms, one mole. The value printed above, 12.011 Da, is a shade higher because about one carbon atom in a hundred is carbon-13. Plants, rocks and oceans take up the two isotopes in slightly different proportions, so the printed weight is a conventional value.

The black core of a pencil is graphite, not lead, and the slipperiness that lets it mark paper also makes graphite a lubricant. Most commercial graphite is not mined: it is baked from petroleum coke, the tar left after crude oil is refined, in an oxygen-free oven. Coke, made by heating soft coal in a sealed oven, is used in large amounts to make steel.

Little natural graphite goes to stationery: the US Geological Survey lists batteries, brake linings, lubricants, powdered metals, refractories and steelmaking as its major uses in 2024. Soot from incomplete burning becomes carbon black, most of which ends up in rubber. Industrial diamond is now largely manufactured: US plants made 160 million carats of grit and powder in 2024, and no natural industrial stone was mined there.

Properties

Properties of carbon Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	6
Standard atomic weight	12.011(2) Da	Da	IUPAC/CIAAW 2024
Melting point	3550 °C	°C	PubChem: 3823 K
Boiling point	3825 °C	°C	PubChem: 4098 K
Density	2.2670 g/cm³	g/cm³
Electronegativity (Pauling)	2.55
Electron configuration	1s² 2s² 2p²
Phase at 25 °C	solid

The standard atomic weight of this element is an interval in nature, from 12.0096 to 12.0116 Da; the value shown is IUPAC's conventional abridged value.

Melting and boiling figures are PubChem's values at standard pressure.

Safety

Solid carbon, whether a pencil core, a lump of charcoal or a cut diamond, is safe to handle, and its record’s cautions are for powder. PubChem’s entry carries the signal word Warning for serious eye irritation and possible respiratory irritation, though two fifths of the reports behind it find no hazard at all. Years of breathing the dust at work affect the lungs, and the World Health Organization’s cancer agency classes fine airborne carbon black as able to cause cancer. Powdered carbon burns readily, and anyone handling it should read the PubChem safety summary and the NIOSH card.

Questions

Why does carbon form so many compounds?

Because every carbon atom makes four bonds, often one to another carbon atom, and carbon-carbon single bonds are very strong. Chains and rings therefore hold together, with hydrogen, oxygen and nitrogen attached along them. Los Alamos calls carbon unique among the elements for the number and variety of compounds it forms, and notes that silicon, directly below it, cannot hold long chains together.

Why are diamond and graphite so different if both are pure carbon?

Only the arrangement of the atoms differs. In diamond the single bonds run through the crystal in three dimensions, which makes it one of the hardest substances known and denser than graphite. PubChem's record puts diamond at 3.15 to 3.53 times the density of water, graphite at 1.9 to 2.3. In graphite the atoms lie in sheets, strongly bonded within a sheet and barely held between sheets, so the sheets slide over one another.

Is pencil lead actually lead?

No. The black core of a pencil is graphite, a form of carbon, and contains no lead. Within a graphite sheet the atoms are held by strong bonds, but between sheets there are only London dispersion forces, the weak attraction between neighbouring electron clouds. Writing peels whole sheets away, and the flakes left on the paper are the mark.

How does carbon dating work?

Cosmic rays keep making carbon-14 in the upper atmosphere, so its level in the environment stays about constant and living things take it in. After death nothing replaces it, and the carbon-14 already there decays to nitrogen-14; its half-life is 5,730 years, the figure Jefferson Lab gives in the PubChem record. A sample holding half the natural amount died about 5,730 years ago.

What is the melting point of carbon?

Not a single number, and at atmospheric pressure not really a melting point. Los Alamos gives 3550 °C for diamond. PubChem gives a boiling point of 3825 °C at standard pressure. Los Alamos gives 3800 °C for the same change and marks it sublimation: solid carbon passes straight to vapour rather than melting. Which number to quote depends on which form of carbon, and which pressure, you mean.

Compounds containing carbon

CO Carbon monoxide 2 atoms, 1 triple bond.
CO₂ Carbon dioxide 3 atoms, 2 double bonds, linear.
C₂H₂ Acetylene 4 atoms, 3 bonds.
CH₂O Formaldehyde 4 atoms, 3 bonds.
CaCO₃ Calcium carbonate 5 atoms in one formula unit of an ionic lattice.
CCl₄ Carbon tetrachloride 5 atoms, 4 single bonds, tetrahedral.
CHCl₃ Chloroform 5 atoms, 4 single bonds, tetrahedral.
CCl₂F₂ Dichlorodifluoromethane 5 atoms, 4 single bonds, tetrahedral.

All 124 compounds containing carbon

CH₂Cl₂ Dichloromethane 5 atoms, 4 single bonds, tetrahedral.
HCOOH Formic acid 5 atoms, 4 bonds.
CH₄ Methane 5 atoms, 4 single bonds, tetrahedral.
H₂CO₃ Carbonic acid 6 atoms, 5 bonds.
C₂H₄ Ethylene 6 atoms, 5 bonds.
CH₃OH Methanol 6 atoms, 5 single bonds.
NaHCO₃ Sodium bicarbonate 6 atoms in one formula unit of an ionic lattice.
Na₂CO₃ Sodium carbonate 6 atoms in one formula unit of an ionic lattice.
C₂H₃Cl Vinyl chloride 6 atoms, 5 bonds.
CH₃CHO Acetaldehyde 7 atoms, 6 bonds.
C₂H₄O Ethylene oxide 7 atoms, 7 single bonds.
C₂H₂F₄ 1,1,1,2-Tetrafluoroethane 8 atoms, 7 single bonds.
CH₃COOH Acetic acid 8 atoms, 7 bonds.
C₂H₆ Ethane 8 atoms, 7 single bonds.
H₂C₂O₄ Oxalic acid 8 atoms, 7 bonds.
CH₃COONa Sodium acetate 8 atoms in one formula unit of an ionic lattice.
CO(NH₂)₂ Urea 8 atoms, 7 bonds.
CH₃OCH₃ Dimethyl ether 9 atoms, 8 single bonds.
C₂H₅OH Ethanol 9 atoms, 8 single bonds.
C₂H₄O₃ Glycolic acid 9 atoms, 8 bonds.
C₃H₆ Propylene 9 atoms, 8 bonds.
C₄H₆ 1,3-Butadiene 10 atoms, 9 bonds.
C₃H₆O Acetone 10 atoms, 9 bonds.
C₂H₆O₂ Ethylene glycol 10 atoms, 9 single bonds.
C₂H₅NO₂ Glycine 10 atoms, 9 bonds.
C₃H₈ Propane 11 atoms, 10 single bonds.
C₂H₅COOH Propionic acid 11 atoms, 10 bonds.
C₃H₇OH 1-Propanol 12 atoms, 11 single bonds.
C₆H₆ Benzene 12 atoms, 12 bonds.
C₃H₈O Isopropyl alcohol 12 atoms, 11 single bonds.
C₃H₆O₃ Lactic acid 12 atoms, 11 bonds.
C₄H₄N₂O₂ Uracil 12 atoms, 12 bonds.
C₃H₇NO₂ Alanine 13 atoms, 12 bonds.
C₄H₈O Butanone 13 atoms, 12 bonds.
C₄H₅N₃O Cytosine 13 atoms, 13 bonds.
C₆H₅OH Phenol 13 atoms, 13 bonds.
C₃H₈O₂ Propylene glycol 13 atoms, 12 single bonds.
(CH₃)₃N Trimethylamine 13 atoms, 12 single bonds.
C₆H₅CHO Benzaldehyde 14 atoms, 14 bonds.
C₄H₁₀ Butane 14 atoms, 13 single bonds.
C₃H₇COOH Butyric acid 14 atoms, 13 bonds.
C₃H₇NO₂S Cysteine 14 atoms, 13 bonds.
CH₃COOC₂H₅ Ethyl acetate 14 atoms, 13 bonds.
C₃H₈O₃ Glycerol 14 atoms, 13 single bonds.
C₄H₁₀ Isobutane 14 atoms, 13 single bonds.
C₄H₉OH 1-Butanol 15 atoms, 14 single bonds.
C₅H₅N₅ Adenine 15 atoms, 16 bonds.
C₆H₅COOH Benzoic acid 15 atoms, 15 bonds.
C₂H₅OC₂H₅ Diethyl ether 15 atoms, 14 single bonds.
C₄H₆O₅ Malic acid 15 atoms, 14 bonds.
C₃H₆N₆ Melamine 15 atoms, 15 bonds.
C₅H₆N₂O₂ Thymine 15 atoms, 15 bonds.
C₇H₈ Toluene 15 atoms, 15 bonds.
C₅H₅N₅O Guanine 16 atoms, 17 bonds.
C₇H₆O₃ Salicylic acid 16 atoms, 16 bonds.
C₈H₈ Styrene 16 atoms, 16 bonds.
C₄H₆O₆ Tartaric acid 16 atoms, 15 bonds.
C₅H₄N₄O₃ Uric acid 16 atoms, 17 bonds.
C₅H₉N₃ Histamine 17 atoms, 17 bonds.
C₅H₁₂ Pentane 17 atoms, 16 single bonds.
C₆H₁₂ Cyclohexane 18 atoms, 18 single bonds.
C₃H₈NO₅P Glyphosate 18 atoms, 17 bonds.
C₁₀H₈ Naphthalene 18 atoms, 19 bonds.
C₅H₉NO₄ Glutamic acid 19 atoms, 18 bonds.
C₈H₈O₃ Methyl salicylate 19 atoms, 19 bonds.
C₅H₈NO₄Na Monosodium glutamate 19 atoms in one formula unit of an ionic lattice.
C₈H₈O₃ Vanillin 19 atoms, 19 bonds.
C₈H₉NO₂ Acetaminophen 20 atoms, 20 bonds.
C₆H₈O₆ Ascorbic acid 20 atoms, 20 bonds.
C₆H₁₄ Hexane 20 atoms, 19 single bonds.
C₄H₁₁N₅ Metformin 20 atoms, 19 bonds.
C₉H₈O₄ Aspirin 21 atoms, 21 bonds.
C₆H₈O₇ Citric acid 21 atoms, 20 bonds.
C₈H₁₁NO₂ Dopamine 22 atoms, 22 bonds.
C₆H₁₃NO₂ Isoleucine 22 atoms, 21 bonds.
C₆H₁₃NO₂ Leucine 22 atoms, 21 bonds.
C₉H₁₁NO₂ Phenylalanine 23 atoms, 23 bonds.
C₈H₁₀N₄O₂ Caffeine 24 atoms, 25 bonds.
C₆H₁₂O₆ Fructose 24 atoms, 24 single bonds.
C₆H₁₂O₆ Glucose 24 atoms, 24 single bonds.
C₉H₁₁NO₃ Tyrosine 24 atoms, 24 bonds.
C₁₀H₁₂N₂O Serotonin 25 atoms, 26 bonds.
C₉H₁₃NO₃ Epinephrine 26 atoms, 26 bonds.
C₈H₁₈ Isooctane 26 atoms, 25 single bonds.
C₁₀H₁₆ Limonene 26 atoms, 26 bonds.
C₁₀H₁₄N₂ Nicotine 26 atoms, 27 bonds.
C₈H₁₈ Octane 26 atoms, 25 single bonds.
C₁₁H₁₂N₂O₂ Tryptophan 27 atoms, 28 bonds.
C₁₂H₁₇NO DEET 31 atoms, 31 bonds.
C₁₀H₂₀O Menthol 31 atoms, 31 single bonds.
C₁₄H₁₄O₃ Naproxen 31 atoms, 32 bonds.
C₁₅H₁₆O₂ Bisphenol A 33 atoms, 34 bonds.
C₁₃H₁₈O₂ Ibuprofen 33 atoms, 33 bonds.
C₁₃H₁₆N₂O₂ Melatonin 33 atoms, 34 bonds.
C₁₅H₁₁I₄NO₄ Thyroxine 35 atoms, 36 bonds.
C₁₃H₂₁NO₃ Albuterol 38 atoms, 38 bonds.
C₂₀H₁₄O₄ Phenolphthalein 38 atoms, 41 bonds.
C₁₄H₁₈N₂O₅ Aspartame 39 atoms, 39 bonds.
C₁₄H₂₂N₂O Lidocaine 39 atoms, 39 bonds.
C₁₇H₂₁NO Diphenhydramine 40 atoms, 41 bonds.
C₁₇H₁₈F₃NO Fluoxetine 40 atoms, 41 bonds.
C₁₆H₁₈N₂O₄S Penicillin G 41 atoms, 43 bonds.
C₁₆H₁₉N₃O₅S Amoxicillin 44 atoms, 46 bonds.
C₁₈H₂₄O₂ Estradiol 44 atoms, 47 bonds.
C₁₂H₂₂O₁₁ Lactose 45 atoms, 46 single bonds.
C₁₂H₂₂O₁₁ Sucrose 45 atoms, 46 single bonds.
C₁₀H₁₆N₅O₁₃P₃ Adenosine triphosphate 47 atoms, 49 bonds.
C₂₀H₂₄N₂O₂ Quinine 48 atoms, 51 bonds.
C₁₉H₂₈O₂ Testosterone 49 atoms, 52 bonds.
C₁₆H₃₂O₂ Palmitic acid 50 atoms, 49 bonds.
C₂₁H₃₀O₂ Progesterone 53 atoms, 56 bonds.
C₁₈H₃₄O₂ Oleic acid 54 atoms, 53 bonds.
C₂₁H₃₀O₅ Cortisol 56 atoms, 59 bonds.
C₂₇H₄₄O Cholecalciferol 72 atoms, 74 bonds.
C₂₇H₄₆O Cholesterol 74 atoms, 77 bonds.
C₃₃H₃₅FN₂O₅ Atorvastatin 76 atoms, 79 bonds.