Carbon

I

Introduction

Carbon, nonmetallic chemical element, known by the symbol C, that is the fundamental building block of material in living organisms and is important to many industries. Carbon occurs in nature in nearly pure form in diamond and graphite. It is also the major component of coal, petroleum, asphalt, limestone, and most materials made by plants and animals. The name carbon is derived from the Latin word carbo, meaning charcoal, a material that is composed primarily of carbon.

A carbon atom can chemically combine with atoms of other elements, as well as with other carbon atoms, to form molecules. Molecules that contain two or more elements make up compounds. Carbon can form more compounds than can any other element except hydrogen.

Carbon is present in all substances known as organic compounds (see Chemistry, Organic). Originally, scientists used the term organic compounds for materials that could only be obtained from living or dead organisms. Today chemists consider nearly any compound that contains carbon to be organic, whether they obtain it from organisms or synthesize it in a laboratory or in factories. Compounds that do not contain carbon are called inorganic compounds.

Carbon atoms form part or all of the backbone for the major molecules of all living things on Earth, including sugars, proteins, fats, and deoxyribonucleic acids (DNA), the molecules that carry the genetic code of living organisms. Many of the materials that we use in everyday life contain carbon-rich organic compounds. For instance, we wear clothing made of organic compounds—either natural fibers, such as wool, silk, or cotton; or synthetic ones, such as nylon or polyester. We construct our houses and furnishings from organic materials, such as wood and plastics. We burn carbon-rich fossil fuels, including gasoline, natural gas, and coal, for heat and energy. In addition, we use organic compounds as pesticides and medicines, and the foods we eat are carbon compounds.

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II

Properties

Of all the elements, carbon is the only one suitable for building the variety of molecules necessary to sustain life. Carbon atoms can attach to each other to form chains, rings, or a crystal mesh. The chains may be thousands of carbon atoms long and either linear or branched, and the rings usually contain from three to six carbon atoms. Most organic compounds contain many carbon-hydrogen bonds. Some of the other elements that bond to carbon include oxygen, nitrogen, fluorine, chlorine, bromine, iodine, sulfur, and phosphorus.

A

Isotopes

Every carbon atom contains six positively charged particles called protons in its nucleus and six or more neutral particles called neutrons. The carbon atom’s nucleus is surrounded by six negatively charged electrons. The number of neutrons in a carbon atom’s nucleus determines which isotope it is. Isotopes are atoms of the same element that have different numbers of neutrons in the nucleus. Three different isotopes of carbon exist in nature. The important isotopes of carbon are carbon-12, carbon-13, and carbon-14. Scientists identify them by their mass number, which is the sum of the number of protons and neutrons in an atom. Carbon-12 contains six protons and six neutrons, carbon-13 contains six protons and seven neutrons, and carbon-14 contains six protons and eight neutrons.

In nature, carbon-12 accounts for about 98.89 percent of all carbon. Carbon-13 has a natural abundance of 1.11 percent, and the amount of carbon-14 is negligible. The atomic mass of carbon is 12.011 atomic mass units (AMU), which is the average mass of the isotopes of carbon based on their abundance.

Scientists have found some important uses for the less abundant isotopes of carbon. The nucleus of carbon-13 is magnetic. This property enables scientists to detect nuclei of carbon-13 atoms using a technique called nuclear magnetic resonance (NMR). By detecting the location of carbon-13 atoms in carbon-based molecules, scientists can learn about the structure of these molecules. Carbon-14 is radioactive, that is, its nucleus is unstable and can spontaneously change into the nucleus of another element (see Radioactivity). In a given sample, half of the carbon-14 nuclei will disintegrate in about 5,730 years. Living organisms constantly replenish carbon in their systems, so that the amount of carbon-14 remains constant as long as an organism is alive. Knowing the original amount of carbon-14 in organisms, scientists can measure the amount of carbon-14 that has disintegrated in a fossilized organism and determine the amount of time that has passed since it died. This technique for determining the age of fossils is called carbon dating.

B

Bonding

As with all atoms, the electrons in a carbon atom reside in layers, or shells, around the nucleus. Carbon atoms have two electrons in their inner shell, and this shell can only contain two electrons, so it is full. Carbon atoms have four outer, or valence, electrons in their next shell. This outer electron shell can hold eight electrons, and atoms in general are much more stable when their outer shell is full. To obtain a full outer shell, carbon atoms form four covalent bonds with other atoms. A covalent bond is a bond formed when two atoms share a pair of electrons. When two atoms share one pair of electrons, the covalent bond is called a sigma bond and it holds the electrons tightly between the two atoms. One pair of shared electrons is also called a single bond. When two atoms share two pairs of electrons (creating a double bond), the first shared pair forms a sigma bond, while the second pair forms a pi bond. The pi bond does not hold electrons as tightly as the sigma bond holds the first pair. When two atoms share three pairs of electrons (creating a triple bond), two of the bonds are pi bonds. Electrons in pi bonds are much more reactive than are electrons in sigma bonds. That is, pi electrons more easily split away from the bond and create bonds with other atoms, adding those atoms to the molecule.

Carbon atoms can bond together in chains, rings, and meshlike networks. If a carbon atom bonds with four identical atoms, those atoms will be equally distant from each other—at the tips of an imaginary tetrahedron, or a pyramid with a triangular base. Any two of the bonds form an angle of 109.5° when carbon is in a tetrahedral form.

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