Carbon

Carbon is the basis of all life on Earth. Every living thing, every plant, animal, bacterium and fungus, is built from carbon atoms. It is also remarkable because it comes in completely different forms: soft black pencil graphite and sparkling hard diamonds are both made of nothing but carbon.

  • Atomic Number66 protons, 6 electrons
  • Atomic Mass12.011 uDefined the atomic mass unit
  • State at Room TempSolidsolid, diamond or graphite
  • Density2.2670 g/cm³Varies: graphite light, diamond dense
  • Melting / Boiling3549.8°C / 3824.8°CHighest melting point of non-metals
  • DiscoveredAncientKnown since ancient times

How does carbon's mass compare to nearby elements?

Carbon sits near the lighter end of the periodic table, between boron and nitrogen.

Atomic Mass Comparison
Hydrogen1.0 u
Carbon12 u
Nitrogen14 u
Oxygen16 u
Iron56 u

Carbon at 12 atomic mass units is so convenient that scientists defined the atomic mass unit itself relative to carbon-12. One carbon atom weighs twelve times a hydrogen atom but just over one fifth of an iron atom.

What is carbon?

Carbon is a non-metal in Group 14 of the periodic table. It is unique because each carbon atom can bond to four other atoms at once, including other carbon atoms, allowing it to form chains, rings and cages of almost unlimited complexity. This ability to build enormous molecules is the reason carbon is the chemical backbone of all known life. No other element comes close to carbon's ability to create diverse and complex structures.

Carbon gets its name from the Latin word carbo, meaning coal or charcoal. Charcoal, made by burning wood without much air, is largely carbon, and humans have used it since the Stone Age for fire, cave drawings and smelting metals. The symbol C comes from the Latin name. Different pure forms of carbon are called allotropes, and they include materials as different as diamond, graphite and the microscopic buckyballs discovered in 1985.

Fact Diamonds and pencil graphite are both made of pure carbon atoms, but arranged in completely different patterns. In diamond, each carbon bonds to four neighbours in a rigid 3D cage, making it the hardest natural material known. In graphite, carbon forms flat sheets that slide over each other, which is what makes graphite slippery and useful for writing.

Where you find carbon

In space

Carbon is the fourth most abundant element in the universe by mass. Inside large stars, three helium nuclei fuse together to form one carbon nucleus, a reaction called the triple-alpha process that created almost all the carbon in existence. The giant planets Jupiter and Saturn contain methane, a carbon compound. Saturn's moon Titan has lakes of liquid methane and an atmosphere rich in carbon-based molecules.

On Earth

Carbon is everywhere on Earth, from the air above you to the rock beneath your feet. It is the second most abundant element in the human body by mass.

  • The atmosphere. Carbon dioxide (CO₂) makes up approx. 0.04% of the air. Plants absorb it during photosynthesis; animals, volcanoes and burning fuels release it.
  • Coal, oil and natural gas. These fossil fuels are the compressed remains of ancient organisms, containing enormous stores of carbon built up over hundreds of millions of years.
  • Limestone and chalk. Calcium carbonate (CaCO₃) is the main component of limestone, marble, chalk and the shells of sea creatures.
  • Living things. Every protein, fat, carbohydrate and DNA molecule in your body contains carbon atoms. Life as we know it is essentially carbon chemistry.

How we use carbon

  • Diamonds. Diamond is the hardest natural material. Industrial diamonds cut glass and drill rock; gem diamonds are treasured in jewellery. Most industrial diamonds are now made artificially.
  • Steel making. Adding a small percentage of carbon to iron makes steel, far stronger than pure iron. The exact amount of carbon controls whether the steel is hard and brittle or tough and flexible.
  • Carbon fibre. Thin fibres of pure carbon, woven into sheets and set in resin, make a material stronger than steel but many times lighter. Used in racing cars, aircraft and sports equipment.
  • Graphite and pencils. Pencil "lead" is actually graphite. When you write, thin layers of graphite flake off and stick to the paper. Graphite is also used as a dry lubricant and in battery electrodes.
  • Fossil fuels. Coal, oil and natural gas are mostly carbon compounds. Burning them releases energy, but also carbon dioxide: the main greenhouse gas behind climate change.
Did you know? Carbon-14 is a radioactive form of carbon that decays at a very predictable rate. Scientists use this to date ancient objects, a technique called radiocarbon dating. By measuring how much carbon-14 remains in a piece of wood, bone or cloth, archaeologists can tell how old it is, up to approx. 50,000 years old.

How it was discovered

Carbon has been known since prehistoric times as charcoal, soot and diamond. Ancient Egyptians used carbon black as a pigment. Diamond has been treasured since at least 2500 BCE. The recognition that these different materials were all forms of the same element came much later. In 1772, Antoine Lavoisier demonstrated that diamonds burned in oxygen to produce only carbon dioxide, proving diamond was a form of carbon. In 1779, Carl Scheele showed graphite did the same thing. By the end of the 18th century, chemists understood that charcoal, graphite and diamond were all forms of one element.

Deeper dive: allotropes of carbon and organic chemistry

Carbon exists in several structural forms called allotropes, each with dramatically different properties. Diamond has every carbon bonded to four others in a tetrahedral cage: the hardest natural material. Graphite has flat sheets of carbon in hexagonal rings, slippery and electrically conducting, used in batteries and pencils. Buckminsterfullerene (C₆₀) is a hollow ball of 60 carbon atoms shaped like a football, discovered in 1985. Graphene is a single layer of graphite one atom thick: the strongest material ever measured by weight.

Organic chemistry is almost entirely the chemistry of carbon. Because carbon forms four bonds and can bond to itself in chains, it creates millions of different molecules. Your body runs on organic chemistry: glucose provides energy, proteins build muscles, DNA stores genes, and fats insulate nerves, all carbon-based.

The carbon cycle is the way carbon moves through the atmosphere, oceans, rocks and living things. Burning fossil fuels releases carbon stored underground over millions of years back into the atmosphere as CO₂, far faster than natural processes can absorb it: the root cause of human-driven climate change.

Carbon is one of the most extraordinary elements on the periodic table: the foundation of all life and of an astonishing range of materials. Moving one step to the right brings us to nitrogen, the gas that makes up most of the air we breathe.