Portraits in Carbon: Forget Oxygen-Carbon May Be the Most Important Element on the Planet

Article excerpt

It's hard to avoid the number six in talking about carbon: It's the sixth element in the periodic table, and normally has six protons and six neutrons in its nucleus and six electrons hovering in loose formation around it. This form, called carbon 12, accounts for 99 percent of all carbon (carbon 13 and 14 account for the rest). As a substance, pure carbon comes in several guises, but among the most important are graphite (which is typically soft, black, slippery, and flaky); diamonds (which can be any of several colors, or colorless, and are as hard as elements come); and amorphous carbon (such as coal and soot). Carbon is abundant in the Earth's crust and biosphere, and as coal costs as little as 1.2 U.S. cents per kilogram. As natural diamonds, however, it can cost thousands or even millions of dollars per gram. The name derives from the Latin carboneum, but is rendered kohlenstoff in German, uhlik in Czech, and sekitan in Japanese.

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Oh, and carbon is the basis of more than 20 million chemical compounds and thus of life as we know it.

Those are the raw, actuarial facts, perhaps carbon's dullest mugshot face. But carbon is more interesting than these facts suggest, and like a quick-change artist has many faces. Here are five more:

Newborn

Carbon is born in dying stars and scattered when they explode. Our own solar system formed from such remnants, a collector hoarding from interstellar estate sales. Carbon represents less than one-tenth of the Earth's crust by mass, but is the only element capable of forming robust single bonds with other atoms at ambient temperatures, which is why it is the chemical backbone of the living world. "Organic chemistry" is carbon chemistry. Carbon is proteins, carbohydrates, hydrocarbons, and DNA. Shakespeare was mostly carbon, and so were his quill, ink, and the paper on which he scratched Hamlet. It's perfume and plastic, trash bags and tennis balls and tree stumps.

It's also natural gas and oil and coal--the fossil fuels that modern civilization runs on.

Much of this was formed during the aptly named Carboniferous Period, a part of the Paleozoic Era from 354 to 290 million years ago. That period saw untold millions of swamp trees with two-meter-diameter trunks die but not rot--in part, some believe, because herbivores, termites, and certain bacteria had not yet evolved to eat lignin (a new plant innovation at the time), which is the glue differentiating wood from leaves. Unable to decompose, the resulting peat piled up in northern Europe, parts of Asia, and in the eastern and midwestern United States. Seas rose and fell, covering the swamps intermittently and cutting off the oxygen that would otherwise fuel organic decay. Pressure and heat over millions of years converted the material to coal, specifically anthracite, the hardest and most energy-rich form, composed of up to 98 percent carbon.

Bituminous coal, softer and younger, is only about 60 percent carbon, but it's lower in sulfur too, and thus emits less acid-rain-forming pollutants when burned. This coal, from places like the Powder River Basin, began as giant ferns and other plants flourishing in the lush effluvial plains of what is now Wyoming about 55 million years ago. Men listening to satellite radio in enormous dump trucks move the obsidian bulk gouged from 15-meter-thick seams to railheads, dispatching 400 million tons every year--40 percent of the coal burned in the United States--to power plants around the country.

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Villain

Which brings us to carbon's dark side. Atmospheric carbon likes to run with pairs of oxygen atoms like some celebrity with arms around identical twins--carbon dioxide (C[O.sub.2]). It is an association born in fire, through the combustion of trees, dung, and fossil fuels. The fossil fuels are the main culprits. Their carbon, largely buried and sequestered until James Watt came up with the steam engine, is being unlocked and flung into the atmosphere at a rate of 7 billion metric tons of carbon every year. …