Where would carbon-based life be without carbon? There are 118 known chemical elements, but carbon is the fourth most abundant and perhaps the most important to human life. Everywhere you look, there's carbon in some form or another, from plastics to proteins to the roads we drive on and the gasoline that fuels our cars. Whether it's a macronutrient, a structural component, or a waste product, carbon is essential to all known forms of life.
Carbon Chemistry
Carbon gets many of its properties from its ability to sustain up to four bonds at a time. If you've ever seen a d4 or a caltrop, you've seen the shape of the bond sites on a carbon atom: four equilateral triangles, arranged into a three-faced pyramid with a triangular base. Every corner has a site that can sustain a chemical bond. Because of that shape, carbon can make branching structures, long zigzagging carbon "backbones," sheets of alternating folds like origami, and even tessellating planes of perfect hexagons.
Carbon is neither very electronegative nor electropositive, which means it neither steals nor throws off electrons when left to its own devices. Its geometry lets it form crystals, and its charge lets it make many different bonds without too much energy. This makes its bonds quite stable over the long haul—although, sadly, diamonds are technically not forever; run out the clock long enough, and on geological timescales, diamonds will eventually degrade into graphite.
Carbon normally has six protons and six neutrons in its nucleus, giving it an atomic mass of 12. But a relatively unstable form of carbon with two extra neutrons, called carbon-14, decays into other elements called "daughter isotopes" over time. The amount of time it takes to decay is so predictable that scientists can use the levels of carbon-14 in a thing to determine how old it is.
The Carbon Cycle
Carbon atoms move around the planet in a process known as the carbon cycle—but really, two carbon cycles are moving together at the same time. Here's how it works:
While Earth's atmosphere is mostly nitrogen and oxygen, a small amount of carbon is found in the atmosphere as carbon dioxide or CO₂. Some atmospheric carbon dissolves into the ocean. Shallow marine environments produce sedimentary deposits that eventually become carbonate rock, such as limestone (made of calcium carbonate, the same stuff in seashells, antacids, and blackboard chalk). When the Earth's crust is recycled, carbonate rock is drawn down into the mantle, melted, and circulated to become new rock at places like deep ocean ridges. Eventually, it becomes sediment that feeds plant growth. This is called the slow carbon cycle.
The fast carbon cycle is concerned with living things. Carbon is found in organic matter (carbon-based compounds associated with living things), from complex proteins down to amino acids, chemicals so simple they're called the "building blocks" of life. Some carbon bound up in the food web stays bound up for a long time. Eventually, though, to dust all things return, and that's how the carbon gets back into the soil and sediment, from where it can either be recycled into the biosphere as new primary productivity or enter the slow carbon cycle.
Carbon dioxide is necessary for plants to grow, perform photosynthesis, and produce the oxygen we breathe. During periods in the past when atmospheric carbon levels were high, some plants grew much larger because the carbon dioxide in the atmosphere was enough to sustain such growth. However, carbon dioxide has a strong enough effect on the atmosphere that to keep it like it is now, we need to keep the carbon dioxide levels from rising too far. Scientists are working on scrubbing carbon dioxide from the air and water, a process that could yield carbon dioxide or solid carbon that is useful to industry.
Atmospheric carbon levels can also affect the ocean and the creatures that live in it. Most creatures have an optimal range of physical conditions: a preferred temperature, oxygen level, and acidity (pH). Carbon dioxide dissolved in water—including saltwater—produces carbonic acid. High levels of atmospheric carbon dioxide create acid rain and depress the pH of surface water, including the ocean. Acid rain can destroy entire populations of species that can't withstand a pH equivalent to tomato juice. Ocean acidification also affects freshwater and saltwater fish, and even benthic or deep-ocean marine creatures such as shellfish and coral reefs. Fossils, ice cores, sediment samples, and other physical historical records tell scientists how much carbon dioxide has been in the atmosphere or dissolved in the ocean, going back a long time into the past.
Earth's carbon cycle is different than that of Venus or Mars, although they may have been similar in the earlier days of the solar system. One thing they all have in common is that they adhere to the greenhouse effect, by which higher densities of gases like CO₂ and methane can absorb energy from the sun, warming the atmosphere.
Neither Venus nor Mars has surface water. Mars is too cold to sustain liquid water on its surface. However, Venus appears to have an atmosphere so dense with carbon dioxide from its numerous volcanoes that a runaway greenhouse effect has heated it enough to melt lead. The Venusian atmosphere is so hot and caustic it has slagged every spacecraft ever sent to the planet's surface. Mars, by comparison, has a carbon dioxide atmosphere so thin it's less than one percent of our atmospheric pressure at sea level. The surface of Mars is so cold that oxygen and CO₂ freeze right out of the sky and fall as snow, leaving carbon dioxide ice caps at Mars' north and south poles.
Industrial Applications of Carbon
Pure carbon is found in nature in several different forms, or allotropes, including graphene, diamond, nanotubes, and fullerenes or "buckyballs." Each form has attributes that make it excel at certain applications. Carbon is also a core component of steel, plastic, and some ceramics.
Graphite
Graphite is a good shield against nuclear radiation because carbon-12 will absorb neutrons and turn into carbon-14, which is why it was used to shield the reactors at Chernobyl. However, because the carbon in graphite can be oxidized, graphite will burn if held at high temperatures long enough (such as those reached inside a nuclear reactor during a meltdown).
With its simple offset crystal lattice, carbon-carbon bonds are weak between layers within a graphite crystal, so graphite tends to flake away along planes. A single-atom-thick layer of graphite is also known as graphene.
Graphene
Graphene's unique electrical properties come from the spooky quantum behavior of electrons. Where diamond has a regular crystal structure, graphene (or at least the Platonic ideal of graphene) is a flat, single-atom-thick sheet of six-sided carbon rings. The hexagonal rings of six carbon atoms that make up graphene are also known as benzene rings. If the carbon's bonding sites aren't fully saturated, the structure will sometimes have an itinerant double bond or two, depending on what substituents the ring has (if any). This phenomenon is known as resonance. With the carbon atoms arranged in a sheet, those moving electron pairs form a shifting moire of resonant bonds as they freely move between pairs of carbon atoms.
The double bond can move because electrons have this irritating tendency to behave like probability clouds, not just point charges: at any given time, a pair of electrons might be here, or they might be over there. Mostly, they'll choose whichever makes it hardest to finish your organic chemistry homework. (I kid, I kid.)
Graphene isn't a semiconductor—but under certain circumstances, it acts like a superconductor. In ideal conditions, graphene offers almost no resistance to the movement of electrons, otherwise known as electrical current. But imperfections in the sheet mean that the original Scotch tape method of making graphene just won't serve for applications that demand great precision, such as photonics and optoelectronics.
Diamond
Diamond is the hardest naturally occurring substance known to science. The Mohs mineral hardness scale describes a mineral's resistance to scratching, and diamond is at the very top of the scale. Industrial applications often use diamond grit as an abrasive. Diamond also has a crystal lattice so rigid that it can withstand incredible forces. Engineers, geologists, physicists, and material scientists sometimes employ a diamond anvil, a tool made from two perfectly flawless diamonds of perhaps 1/3 carat each. Using a diamond anvil, we can achieve pressures on the order of hundreds of gigapascals, the equivalent of millions of Earth atmospheres whose crushing force is directed onto a spot less than a quarter of a millimeter on a side.
Diamond anvils are often made from diamonds created in a lab instead of mined from the ground. Flawless and optically perfect, these crystals are laid down using controlled processes like chemical vapor deposition. This allows scientists to fine-tune the characteristics of a given crystal. Like silicon, diamonds can be "doped" with certain atoms that its rigid crystal lattice will retain, such as nitrogen and boron. Created diamonds can even be made in all the same colors as regular diamonds, which makes them an attractive way to avoid the ethical issues that gave rise to the term "blood diamonds."
Ceramics
Beyond its presence in bioceramics such as seashells, carbon is a crucial part of industrial ceramics, including silicon carbide (SiC), a semiconductor also known as the gemstone moissanite. SiC is found in nature in its gemstone form, but only in places like meteorites and kimberlite. It's also produced industrially by methods like sintering or single crystal growth, which can produce optically perfect synthetic moissanite gemstones.
Polymers
Carbon's unique geometry allows carbon atoms to form long chains called polymers. Polymers are made of monomers linked together. Some carbon polymers, like lipids and petroleum hydrocarbons, take the form of one carbon atom plugged into the next in a single strand. Others are made of more complex molecules joined together, such as polymer plastics like polyethylene, polycarbonate, and PVC. Commercially important biopolymers include wool, silk, and the cellulose fibers that make up linen, cotton, and wood.
One special application of polymer chemistry is a carbon composite, carbon fibers, which double-dip in the polymer category. To start with, carbon atoms are formed into fibrous carbon crystals or a long ribbon of graphene, and then the fibers are bonded to a plastic resin, another carbon polymer. The resulting composite is light, tough, and abrasion-resistant.
Fullerenes
Because carbon is so good at forming sheets and chains, sometimes those sheets get wrapped around and bind to themselves edge-to-edge, like sewing together the sleeve of a shirt. When carbon forms these positively curved (i.e. convex) structures, it is an allotrope called fullerenes or "buckyballs." Fullerenes and Buckyballs are named after Richard Buckminster "Bucky" Fuller, who discovered them and explored their various forms. While their properties are under keen investigation for industrial uses, fullerenes are easy to find—perhaps too easy, since buckyballs and nanotubes are the chief components of lampblack or soot.
Like diamonds, carbon nanotubes are among the strongest materials known. But where diamonds are strong under compression, carbon nanotubes have incredible tensile strength. Because of their structure, nanotubes also have unique electrical properties. They're like a sheet of graphene rolled up and bonded edge to edge into a seamless cylinder. Nanotubes are another important part of many composite materials, including a double-sided tape that uses carbon nanotubes to mimic the setae on a gecko's feet.
Carbon in Living Things
Carbon is a critical part of the structure of all living things, in every branch of the Tree of Life. All the fundamental types of biomolecules require carbon. Proteins and lipids run on carbon, nitrogen, and phosphorus, while carbohydrates, sugars, starches, and even chitin are based on carbon, hydrogen, and oxygen. There's even carbon in the crystal lattice of the mineral hydroxyapatite, found in bones and teeth. Nucleic acids are no exception; RNA and DNA are carbon polymers. DNA has a "backbone" made of ribose molecules (a five-carbon sugar; glucose is a six-carbon sugar) strung together in a row.
What makes DNA so special, among all other biomolecules, is its ability to carry information. DNA is a polymer made of four different monomer units, whose sequence encodes all the instructions necessary to make a living thing that will grow and prosper. Much of the genome codes for proteins, such as their role, effects, and even shape, are a function of their primary sequence. Some of those proteins are even devoted to DNA repair. Most species produce so-called DNA polymerase proteins whose only job is to fix DNA, and to do that repair, they have to cut the genome in the middle of the strand and then stick it back together. This is a workable prospect because of carbon's unique chemical properties.
It's thought that carbon is the only element that could be the foundation of a tree of life like ours. Carbon makes and releases valence bonds easily because its charge is neutral. Silicon is in the same group as carbon, but silicon is much heavier, so it takes more energy to make and break bonds. Conditions where there's enough free energy to permit silicon to polymerize—for instance, the inside of a star—are not known to support life as we understand it. Still, the idea of silicon-based life may ring a bell for fans of science fiction such as Star Trek, where a newly discovered silicon-based alien life form named the Crystalline Entity becomes first a scientific curiosity and then an existential threat. Maybe sticking to carbon is the right idea.
