The Carbon Cycle - How it works



Geochemistry

Chemistry is concerned with the composition, structure, properties, and changes of substances, including elements, compounds, and mixtures. Central to the discipline is the atomic model, or the idea that all matter is composed of atoms, each of which represents one and only one chemical element. An element thus is defined as a substance made up of only one kind of atom, which cannot be broken chemically into other substances. A chemical reaction involves either the bonding of one atom with another or the breaking of chemical bonds between atoms.

Geochemistry brings together geology and chemistry, though as the subdiscipline has matured in the period since the 1940s, its scope has widened to take in aspects of other disciplines and subdisciplines. With its focus on such issues as the recycling of elements between the various sectors of the earth system, especially between living and nonliving things, geochemistry naturally encompasses biology, botany, and a host of earth science subdisciplines, such as hydrology.

BIOGEOCHEMICAL CYCLES.

Among the most significant areas of study within the realm of geochemistry are biogeochemical cycles. These are the changes that a particular element undergoes as it passes back and forth through the various earth systems—particularly between living and nonliving matter. As we shall see, this transition between the worlds of the living and the nonliving is particularly interesting where carbon is concerned.

Along with carbon, five other elements—hydrogen, nitrogen, oxygen, phosphorus, and sulfur—are involved in biogeochemical cycles. With the exception of phosphorus, which plays little part in the atmosphere, these elements move through all four earth systems, including the atmosphere, the biosphere (the sum of all living things as well as formerly living things that have not yet decomposed), the hydrosphere (Earth's water, except for water vapor in the atmosphere), and the geosphere, or the upper part of Earth's continental crust.

Earth systems and biogeochemical cycles are discussed in greater depth within essays devoted to those topics (see Earth Systems and Biogeochemical Cycles). Likewise, the nitrogen cycle is treated separately (see Nitrogen Cycle). The role of hydrogen and oxygen, which chemically bond

THE NAME CARBON COMES FROM THE LATIN WORD FOR CHARCOAL, CARBO. COAL HAS A WIDE VARIETY OF USES, FROM MANUFACTURING STEEL TO GENERATING ELECTRICITY. (© Andrew J. Martinez/Photo Researchers. Reproduced by permission.)
T HE NAME CARBON COMES FROM THE LATIN WORD FOR CHARCOAL, CARBO. COAL HAS A WIDE VARIETY OF USES, FROM MANUFACTURING STEEL TO GENERATING ELECTRICITY . (
© Andrew J. Martinez/Photo Researchers
. Reproduced by permission. )
to form water, is discussed in the context of the hydrosphere (see Hydrologic Cycle).

Elements and Compounds

We have referred to elements and compounds, which are essential to the study of chemistry; now let us examine them briefly before going on to the subject of a specific and very important element, carbon. An element is defined not by outward characteristics, though elements do have definable features by which they are known; rather, the true meaning of the term element is discernible only at the atomic level.

Every atom has a nucleus, which contains protons, or subatomic particles of positive electric charge. The identity of an element is defined by the number of protons in the nucleus: for instance, if an atom has only a single proton, by definition it must be hydrogen. An atom with six protons in the nucleus, on the other hand, is always an atom of carbon. Thus, the elements are listed on the periodic table of elements by atomic number, or the number of protons in the atomic nucleus.

ELECTRONS AND CHEMICAL REACTIONS.

While protons are essential to the definition of an element, they play no role in the bonding between atoms, which usually produces chemical compounds. (The reason for this is qualified by the modifier usually, in that sometimes two atoms of the same element may bond as well.) Chemical bonding involves only the electrons, which are negatively charged subatomic particles that spin around the nucleus. In fact, only certain of these fast-moving particles take part in bonding. These are the valence electrons, which occupy the highest energy levels in the atom.

One might say that valence electrons are at the "outside edge" of the atom, though the model of atomic structure, considered only in the briefest form here, is far more complex than that phrase implies. In any case, elements have characteristic valence electron patterns that affect their reactivity, or their ability to bond. Carbon is structured in such a way that it can form multiple bonds, and this feature plays a significant part in its importance as an element.

When an element reacts with another, they join together, generally in a molecule (we will examine some exceptions), to form a compound. Though the atoms themselves remain intact, and an element can be released from a compound, a compound quite often has properties quite unlike those of the original elements. Carbon and oxygen are essential to sustaining life, but when a single atom of one bonds with a single atom of the other, they form a toxic gas, carbon monoxide. And whereas carbon in its elemental form is a black powder and hydrogen and oxygen are colorless, odorless gases, when bonded in the proper proportions and structure, the three create sugar.

Carbon

The name carbon comes from the Latin word for charcoal, carbo. In fact, charcoal—wood or other plant material that has been heated without enough air present to make it burn—is just one of many well-known substances that contain carbon. Others include coal, petroleum, and other fossil fuels, all of which contain hydrocarbons, or chemical compounds built around strings of carbon and hydrogen atoms. Graphite is pure carbon, and coke, a refined version of coal, is very nearly pure. Not everything made of carbon is black, however: diamonds, too, are pure carbon in another form.

Though carbon makes up only a small portion of the known elemental mass in Earth's crust, waters, and atmosphere—just 0.08%, or 1/1,250 of the whole—it is the fourteenth most abundant element on the planet. In the human body, carbon is second only to oxygen in abundance and accounts for 18% of the body's mass. Present in the inorganic rocks of the ground and in the living creatures above it, carbon is everywhere in the earth system.

CARBON BONDING.

There are two elements noted for their ability to form long strings of atoms and seemingly endless varieties of molecules: one is carbon, and the other is silicon, directly below it on the periodic table. Just as carbon forms a vast array of organic compounds, silicon, found in a huge variety of minerals, is at the center of a large number of inorganic compounds. Yet carbon is capable of forming an even greater number of bonds than silicon. (For more about silicon and the silicates, see the entries Minerals and Economic Geology.)

Carbon is distinguished further by its high value of electronegativity, the relative ability of an atom to attract valence electrons. In addition, with four valence electrons, carbon is ideally suited to finding other elements (or other carbon atoms)

A DIAMOND IS AN ALLOTROPE, A CRYSTALLINE FORM, OF CARBON. ESSENTIALLY, IT IS A HUGE MOLECULE COMPOSED OF CARBON ATOMS STRUNG TOGETHER BY COVALENT BONDS. (© V. Fleming/Photo Researchers. Reproduced by permission.
A DIAMOND IS AN ALLOTROPE , A CRYSTALLINE FORM , OF CARBON . E SSENTIALLY , IT IS A HUGE MOLECULE COMPOSED OF CARBON ATOMS STRUNG TOGETHER BY COVALENT BONDS . (
© V. Fleming/Photo Researchers
. Reproduced by permission.
with which to form chemical bonds. Normally, an element does not necessarily have the ability to bond with as many other elements as it has valence electrons, but carbon—with its four valence electrons—happens to be tetravalent, or capable of bonding to four other atoms at once. Additionally, carbon can form not just a single bond but also a double bond or even a triple bond with other elements.

ALLOTROPES OF CARBON.

Carbon has several allotropes—different versions of the same element distinguished by their molecular structure. The first of them is graphite, a soft material that most of us regularly encounter in the form of pencil "lead." Graphite is essentially a series of one-atom-thick sheets of carbon bonded together in a hexagonal pattern, but with only very weak attractions between adjacent sheets.

Then there is that most alluring of all carbon allotropes, diamond. Neither diamonds nor graphite, strictly speaking, are formed of molecules. Their arrangement is definite, as with a molecule, but their size is not: they simply form repeating patterns that seem to stretch on forever. Whereas graphite is in the form of sheets, a diamond is basically a huge "molecule" composed of carbon atoms strung together by what are known as covalent chemical bonds.

Graphite and diamond are both crystalline—solids in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions. (All minerals are crystalline in structure. See Minerals.) A third carbon allotrope, buckminsterfullerene, discovered in 1985 and named after the American engineer and philosopher R. Buckminster Fuller (1895-1983), is also crystalline in form.

Carbon takes yet another form, distinguished from the other three allotropes in that it is amorphous in structure—lacking a definite shape—as opposed to crystalline. Though it retains some of the microscopic structures of the plant cells in the wood from which it is made, charcoal is mostly amorphous carbon. Coal and coke are particularly significant varieties of amorphous carbon. Formed by the decay of fossils, coal was the first important fossil fuel (discussed later in this essay) used to provide heat and power to human societies.

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