Carbon

In the form of chemical compounds, carbon is distributed throughout the world as carbon dioxide gas, CO₂, in the atmosphere and dissolved in all the rivers, lakes and oceans. In the form of carbonates, mostly calcium carbonate (CaCO₃), it occurs as huge rocky masses of limestone, marble, and chalk. In the form of hydrocarbons, it occurs as great deposits of natural gas, petroleum, and coal. Coal is important not only as a fuel, but because it is the source of the carbon that we dissolve in molten iron to make steel.

All plants and animals on Earth contain a substantial proportion of carbon. After hydrogen and oxygen, carbon is the most abundant element in the human body, making up 10.7% of all the body's atoms.

Carbon is found as the free (uncombined) element in three different allotropic forms-different geometrical arrangements of the atoms in the solid. The two crystalline forms (forms containing very definite atomic arrangements) are graphite and diamond. Graphite is one of the softest known materials, while diamond is one of the hardest.

There is also a shapeless, or amorphous, form of carbon in which the atoms have no particular geometric arrangement. Carbon black, a form of amorphous carbon obtained from smoky flames, is used to make rubber tires and inks black. Charcoal—wood or other plant material that has been heated in the absence of enough air to actually burn—is mostly amorphous carbon, but it retains some of the microscopic structure of the plant cells in the wood from which is was made. Activated charcoal is charcoal that has been steam-purified of all the gummy wood-decomposition products, leaving porous grains of pure carbon that have an enormous microscopic surface area. It is estimated that one cubic inch of activated charcoal contains 200,000 sq ft (18,580 m²) of microscopic surface. This huge surface has a stickiness, called adsorption, for molecules of gases and solids; activated charcoal is therefore used to remove impurities from water and air, such as in home water purifiers and in gas masks.