CARBON Ordinary coal is a form of amorphous carbon

Atomic Number: 6
Atomic Mass Number: 12.011
Atomic Radius:
Electron Affinity:
First Ionization Energy:
Second Ionization Energy:
Electron Confguration: 1s²2s²2p⁴

Carbon is an element truly in a class by itself. With four valence electrons, it is four electrons short of having a noble gas electron configuration. One would expect carbon to use each of the four valence electrons in the formation of chemical bonds and in this way achieve an octet of electrons. This is indeed the case for many carbon compounds. The true uniqueness of carbon comes, however, with the ability of carbon atoms to bond with themselves. This is a factor which can be attributed to the strength of the carbon-carbon bond. . The chemistry of carbon compounds is so diverse that an entire field of organic chemistry is devoted to the chemistry of this one element.

Pure carbon occurs as several different allotropes, or substances composed entirely of carbon atoms but differ in the way the atoms are arranged. For example, coal is composed entirerely of carbon. Coal is is an amphorous substance, which means that the it has no long-range order. Diamond and graphite are two crystalline allotropes of carbon. The fullerenes, a relatively recent discovery, are molecular substances composed entirely of carbon.

Diamond and Lonsdaleite

Diamond is one allotrope of carbon. In the diamond structure, each carbon atom is covalently bonded to four others and has a tetrahedal geometry. In essence, a diamond is one large molecule. This reflected in the chemical properties of diamond. Diamond is a very hard material and has a very high melting point. Diamond is denser than graphite and has a density of 3.51 grams per cubic centimeter. Related to the diamond structure is the silicon carbide structure. In the silicon carbide structure each atom is tetrahedrally bonded to four atoms of another type.

		Two models of the diamond structure
view an animation of this model	view an animation of this model

Lonsdaleite is similar to diamond in that each carbon atom is connected to four others. However, in the case of lonsdaleite, the atoms are arranged to give a hexagonal rather than cubic structure. Lonsdaleite is increadibly rare and was originally discovered in meteroites.

		Two views of the lonsdaleite structure from the side (left) and from the top (right)
view an animation of this model	view an animation of this model

Graphite

Graphite is a second allotrope of carbon. As is the case with diamond, pure graphite contains only carbon atoms. However, in the case of graphite, the atoms are arranged in layers. Within each layer, the atoms are joined to form six-membered rings and each carbon atom is connected to three others. Of the three bonds, two are single bonds and one is a double bond. Therefore the overall bond order is 1.33. Graphite is a soft, slippery substance, attributed to the fact that the layers are not chemically bonded and can slide over one another. An unusual property of graphite is that it is electrically conductive. Graphite is less dense than carbon and has a density of 2.22 grams per cubic centimeter.

One possible resonacne structure for one layer of the graphite structure (left). A three-dimensional model of the graphite structure (right).

Fullerenes

Diamond and graphite used to be considered the only allotropes of carbon. Discovered only recently, Fullerenes represent a third. One of the first to be discovered wasC₆₀, shown below. This is a spherical moelcule shaped like a soccer ball. As in graphite, each carbon atom is bonded to three others. However, the fullerenes contain five-membered as well as six-membered rings, and it is the five-membered rings that are responsible for the curvature of the molecule.

Two views of the C60 molecule. Five-membered rings are shown in violet. The second most common fullerene molecule is C70. The C70 molecule is not spherical like C60 and can be viewed as a stretched C60 molecule. All fullerenes have 12 five membered rings, but difering numbers of six membered rings. In the case of C70, the ten additional atoms produce an additional ring of six-membered rings, stretching the molecule into an oblong shape.

Two views of the C70 molecule. Five-membered rings are shown in violet.

Carbon forms a number of binary compounds. Carbon readily forms bonds with hydrogen, oxygen, nitrogen, sulfur, and the halogens. Since carbon has four valence electrons, it can achieve an octet of electrons by using each of the four electrons in the formation of chemical bonds. Such structures have a tetrahedral geometry. Examples include methane and carbon tetrachloride. Carbon also forms a series of compounds with oxygen. These include carbon dioxide, carbon monoxide, and carbon suboxide. Carbon dioxide is formed in huge quantities by the combustion of fossil fuels. It is a colorless and odorless gas. Dry ice is composed of frozen carbon dioxide. Under standard conditions of temperature and pressure dry ice passes from a solid directly to a gas without going through a liquid phase, a process known as sublimation. In the carbon dioxide molecule, the oxygen atoms are each joined to the carbon atom by double bonds. The carbon atom is sp hybridized and the molecule has a linear shape. Carbon monoxide is formed by the incomplete combustion of fossil fuels. In the carbon monoxide molecule the atoms are joined by a triple bond and both atoms are sp hybridized. Like carbon dioxode, carbon monoxode is a colorless, odorless gas. It is, however, extremely toxic because it combines with hemoglobin many times better than oxygen. Carbon monoxide is also a very good ligand, and combines binds with manu metals to form compounds called metal carbonyls. Carbon suboxide is a third, and much less common, carbon-oxygen compound. It is believed to have a linear shape.

Just as oxygen gains two electrons to form the oxide ion and nitrogen gains three electrons to form the nitride ion, carbon forms anions called carbides. Carbides are essentially ionic compounds that form colorless, transparent crystals. There are two types of carbides, as summarized in the table below. The first type of carbides are referred to as dicarbides or acetylides. Dicarbides are formed mostly by the alkali metals and alkaline earth metals. A relatively common example is calcium carbide. Dicarbides react with water to release acetylene gas and the corresponding metal hydroxide. The carbon-carbon bond distance in carbides is quite short and consitent with a triple bond; the carbide ion is isoelectronic with the diatomic nitrogen molecule and the cyanide ion. A second type of carbides are referred to as methides. In contrast to the dicarbides, the methides react with water to produce methane gas and the corresponding metal hydroxide. Examples include aluminum and beryllium carbide.

The reaction of calcium carbide with water produces acetylene, a flammable gas See a video of this reaction on the reactions page