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Facts About Carbon

Carbon
Carbon
Credit: general-fmv, Andrei Marincas | Shutterstock

Carbon is an incredible element. Arrange carbon atoms in one way, and they become soft, pliable graphite. Re-jigger the arrangement, and — presto! — the atoms form diamond, one of the hardest materials in the world.

Carbon is also the key ingredient for most life on Earth; the pigment that made the first tattoos; and the basis for technological marvels such as graphene, which is a material stronger than steel and more flexible than rubber. [See Periodic Table of the Elements]

Carbon occurs naturally as carbon-12, which makes up almost 99 percent of the carbon in the universe; carbon-13, which makes up about 1 percent; and carbon-14, which makes up a minuscule amount of overall carbon but is very important in dating organic objects. 

Just the facts

  • Atomic Number (number of protons in the nucleus): 6
  • Atomic Symbol (on the Periodic Table of Elements): C
  • Atomic Weight (average mass of the atom): 12.0107
  • Density: 2.2670 grams per cubic centimeter
  • Phase at Room Temperature: Solid
  • Melting Point: 6,422 degrees Fahrenheit (3,550 degrees C)
  • Boiling Point: 6,872 F (3,800 C) (sublimation)
  • Number of isotopes: 15 total; two stable isotopes, which are atoms of the same element with a different number of neutrons.
  • Most common isotopes: carbon-12 (6 protons, 6 neutrons and 6 electrons) and carbon-13 (6 protons, 7 neutrons and 6 electrons)

Carbon: From stars to life

As the sixth-most abundant element in the universe, carbon forms in the belly of stars in a reaction called the triple-alpha process, according to the Swinburne Center for Astrophysics and Supercomputing.  

In older stars that have burned most of their hydrogen, leftover helium accumulates. Each helium nucleus has two protons and two neutrons. Under very hot temperatures — greater than 100,000,000 Kelvin (179,999,540.6 F) — the helium nuclei begin to fuse, first as pairs into unstable 4-proton beryllium nuclei, and eventually, as enough beryllium nuclei blink into existence, into a beryllium plus a helium. The end result: Atoms with six protons and six neutrons — carbon.

electron cloud figure of a carbon atom
While scientists sometimes conceptualize electrons spinning around an atom's nucleus in a defined shell, they actually fly around the nucleus at various distances; this view of the carbon atom can be seen here in two electron cloud figures (bottom), showing the electrons in a single blob (the so-called s-orbital) and in a two-lobed blob or cloud (the p-orbital).
Credit: Physical Review B, DOI: 10.1103/PhysRevB.80.165404

Carbon is a pattern maker. It can link to itself, forming long, resilient chains called polymers. It can also bond with up to four other atoms because of its electron arrangement. Atoms are arranged as a nucleus surrounded by an electron cloud, with electrons zinging around at different distances from the nucleus. Chemists conceive of these distances as shells, and define the properties of atoms by what is in each shell, according to the University of California, Davis. Carbon has two electron shells, with the first holding two electrons and the second holding four out of a possible eight spaces. When atoms bond, they share electrons in their outermost shell. Carbon has four empty spaces in its outer shell, enabling it to bond to four other atoms. (It can also bond stably to fewer atoms by forming double and triple bonds.)

In other words, carbon has options. And it uses them: Nearly 10 million carbon compounds have been discovered, and scientists estimate that carbon is the keystone for 95 percent of known compounds, according to the website Chemistry Explained. Carbon's incredible ability to bond with many other elements is a major reason that it is crucial to almost all life.

Carbon's discovery is lost to history. The element was known to prehistoric humans in the form of charcoal. Carbon as coal is still a major source of fuel worldwide, providing about 30 percent of energy worldwide, according to the World Coal Association. Coal is also a key component in steel production, while graphite, another form of carbon, is a common industrial lubricant.

Carbon-14 is a radioactive isotope of carbon used by archaeologists to date objects and remains. Carbon-14 is naturally occurring in the atmosphere. Plants take it up in respiration, in which they convert sugars made during photosynthesis back into energy that they use to grow and maintain other processes, according to Colorado State University. Animals incorporate carbon-14 into their bodies by eating plants or other plant-eating animals. Carbon-14 has a half-life of 5,730 years, meaning that after that time, half of the carbon-14 in a sample decays away, according to the University of Arizona.

Because organisms stop taking in carbon-14 after death, scientists can use carbon-14's half-life as a sort of clock to measure how long it has been since the organism died. This method works on once-living organisms, including objects made of wood or other plant material.

Who knew?

  • Carbon gets its name from the Latin word carbo, which means "coal."
  • Diamonds and graphite are among the hardest and softest natural materials known, respectively. The only difference between the two is their crystal structure.
  • Carbon makes up 0.032 percent of the Earth's lithosphere (crust and outer mantle) by weight, according to the Encyclopedia of Earth. A rough estimate of the weight of the lithosphere by La Salle University geologist David Smith is 300,000,000,000,000,000,000,000 (or 3*10^23) pounds, making the approximate weight of carbon in the lithosphere 10,560,000,000,000,000,000,000 (or 1.056*10^22) pounds.
  • Carbon dioxide (a carbon atom plus two oxygen atoms) makes up about 0.04 percent of Earth's atmosphere, according to the National Oceanic & Atmospheric Administration (NOAA) — an increase over pre-industrial times, because of the burning of fossil fuels.
  • Carbon monoxide (a carbon atom plus one oxygen atom) is an odorless gas produced from the burning of fossil fuels. Carbon monoxide kills by binding to hemoglobin, the oxygen-carrying compound in the blood. Carbon monoxide bonds to hemoglobin 210 times more strongly than oxygen binds to hemoglobin, effectively crowding out oxygen and suffocating the tissues, according to a 2001 paper in the Journal of the Royal Society of Medicine.   
  • Diamond, the flashiest version of carbon, is formed under great pressure deep in the Earth's crust. The largest gem-quality diamond ever found was the Cullinan diamond, which was discovered in 1905, according to the Royal Collection Trust. The uncut diamond was 3,106.75 carats. The largest gem cut from the stone, at 530.2 carats, is one of the Crown Jewels of the United Kingdom and is known as the Great Star of Africa.
  • The tattoos of Ötzi the Iceman, a 5,300-year-old corpse found frozen in the Alps, were inked from carbon, according to a 2009 study in the Journal of Archaeological Science. Small incisions in the skin were made, and charcoal rubbed in, perhaps as part of an acupuncture treatment.

Ongoing research

Carbon is a long-studied element, but that doesn't mean there isn't more to discover. In fact, the same element that our prehistoric ancestors burned as charcoal may be the key to next-generation tech materials.

In 1985, Rick Smalley and Robert Curl of Rice University in Texas and their colleagues discovered a new form of carbon. By vaporizing graphite with lasers, the scientists created a mysterious new molecule made of pure carbon, according to the American Chemical Society. This molecule turned out to be a soccer-ball-shaped sphere made of 60 carbon atoms. The research team named their discovery the buckminsterfullerene after an architect who designed geodesic domes. The molecule is now more commonly known as the "buckyball." The researchers who discovered it won a Nobel Prize in Chemistry in 1996. Buckyballs have been found to inhibit the spread of HIV, according to a study published in 2009 in the Journal of Chemical Information and Modeling; medical researchers are working to attach drugs, molecule-by-molecule, to buckyballs in order to deliver medicine directly to sites of infection or tumors in the body; this includes research by Columbia University, Rice University and others.  

Since then, other new, pure carbon molecules — called fullerenes — have been discovered, including elliptical-shaped "buckyeggs" and carbon nanotubes with amazing conductive properties. Carbon chemistry is still hot enough to capture Nobel Prizes: In 2010, researchers from Japan and the United States won one for figuring out how to link carbon atoms together using palladium atoms, a method that enables the manufacture of large, complex carbon molecules, according to the Nobel Foundation.

Scientists and engineers are working with these carbon nanomaterials to build materials straight out of science-fiction. A 2010 paper in the journal Nano Letters reports the invention of flexible, conductive textiles dipped in a carbon nanotube "ink" that could be used to store energy, perhaps paving the way for wearable batteries, solar cells and other electronics.

Perhaps one of the hottest areas in carbon research today, however, involves the "miracle material" graphene. Graphene is a sheet of carbon only one atom thick. It's the strongest material known while still being ultralight and flexible. And it conducts electricity better than copper.

Mass-producing graphene is a challenge, though researchers in April 2014 reported that they could make large amounts using nothing but a kitchen blender. If scientists can figure out how to make lots of graphene easily, the material could become huge in tech. Imagine flexible, unbreakable gadgets that also happen to be paper-thin. Carbon has come a long way from charcoal and diamonds, indeed.  

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Stephanie Pappas

Stephanie interned as a science writer at Stanford University Medical School, and also interned at ScienceNow magazine and the Santa Cruz Sentinel. She has a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science writing from the University of California, Santa Cruz.
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