CARBON SYMBOL:C PERIOD:2 GROUP:14(IVA) ATOMICNO:6
CARBON SYMBOL:C PERIOD:2 GROUP:14(IVA) ATOMICNO:6
ATOMICMASS:12.01115amu. VALENCE:4 OXIDATIONSTATE:+2,+4,and– 4 NATURALSTATE:Solid ORIGINOFNAME: Carbon’snameisderivedfromtheLatinwordcarbo,whichmeans,
“charcoal.”
Guide to the Elements |
ISOTOPES:Thereare15isotopesofcarbon,twoofwhicharestable.Stablecarbon-12 makesup98.89%oftheelement’snaturalabundanceintheEarth’scrust,andcar- bon-13makesupjust1.11%ofcarbon’sabundanceintheEarth’scrust.Alltheother isotopesofcarbonareradioactivewithhalf-livesvaryingfrom30nanoseconds(C-21)to 5,730years(C-14).
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p2
Properties All the elements in group 14 have four electrons in their outer valence shell. Carbon exhib-
its more nonmetallic properties than do the others in group 14 and is unique in several ways. It has four forms, called allotropes:
1. Carbon black is the amorphous allotrope (noncrystal form) of carbon. It is produced by heating coal at high temperatures (producing coke); burning natural gas (producing jet black); or burning vegetable or animal matter (such as wood and bone), at high tempera- tures with insufficient oxygen, which prevents complete combustion of the material, thus producing charcoal.
2. Graphite is a unique crystal structure of carbon wherein layers of carbon atoms are stacked parallel to each other and can extend indefinitely in two dimensions as in the shafts of carbon fiber golf clubs. Graphite is also one of the softest elements, making it an excellent dry lubricant.
3. Diamonds are another allotrope whose crystal structure is similar to graphite. Natural diamonds were formed under higher pressure and extreme temperatures. Synthetic dia- monds have been artificially produced since 1955.
4. Fullerenes are another amorphous (no crystal structure) form of carbon that have the basic
formula of C 60 H 60 and are shaped like a soccer ball. (See the “Atomic Structure” section of the book for more on fullerenes.)
The different allotropes of carbon were formed under varying conditions in the Earth, starting with different minerals, temperature, pressure, and periods of time. Once the distinct crystal structures are formed, they are nearly impossible to change.
Carbon-12 is the basis for the average atomic mass units (amu) that is used to determine the atomic weights of the elements. Carbon is one of the few elements that can form covalent bonds with itself as well as with many metals and nonmetals.
192 | The History and Use of Our Earth’s Chemical Elements Each allotropic form of carbon has its own melting point, boiling point, and density. For
instance, the density of the amorphous allotrope is 1.9 g/cm 3 , and it is 2.25 g/cm 3 for graphite and 3.52 g/cm 3 for diamonds.
Characteristics Carbon is, without a doubt, one of the most important elements on Earth. It is the major
element found in over one million organic compounds and is the minor component in min- erals such as carbonates of magnesium and calcium (e.g., limestone, marble, and dolomite), coral, and shells of oysters and clams.
The carbon cycle, one of the most essential of all biological processes, involves the chemi- cal conversion of carbon dioxide to carbohydrates in green plants by photosynthesis. Animals consume the carbohydrates and, through the metabolic process, reconvert the carbohydrates back into carbon dioxide, which is returned to the atmosphere to continue the cycle.
AbundanceandSource Carbon is the 14th most abundant element, making up about 0.048% of the Earth’s crust.
It is the sixth most abundant element in the universe, which contains 3.5 atoms of carbon for every atom of silicon. Carbon is a product of the cosmic nuclear process called fusion, through which helium nuclei are “burned” and fused together to form carbon atoms with the atomic number 12. Only five elements are more abundant in the universe than carbon: hydrogen, helium, oxygen, neon, and nitrogen.
History Carbon was known in prehistoric times in the form of charcoal and later as peat and coal
deposits. Graphite’s name was derived from the Greek word graphein, meaning “to write.” resulting from the fact that graphite was used to make dark marks on paper. At one time, early chemists confused graphite with lead and molybdenite. Diamonds were also known in ancient times. The word diamond is from the Greek word adamas, meaning “the invincible,” which well describes this allotrope of carbon.
Antoine-Laurent Lavoisier (1743–1794) is known as the “father of modern chemistry” because he believed in weighing, measuring, observing, heating, and testing the substances with which he experimented, as well as in keeping accurate records of his findings. He was among the first to experiment with carbon chemistry, and his techniques led to the field of modern quantitative chemistry.
Lavoisier, along with a number of other chemists, pooled their funds and purchased a dia- mond, which they placed in a closed glass jar. Using a magnifying glass, they focused the sun’s rays on the diamond. This produced enough heat to make the diamond disappear. Given that the weight of the glass container that held the diamond was unchanged, Lavoisier determined that the colorless substance in the glass container was a gas—in this case, carbon dioxide. He concluded that the no-longer-visible diamond was the carbon that combined with the oxygen
in the container to form CO 2 . For years it was thought that diamonds were made of carbon atoms, just like graphite and coal, but no one could demonstrate this. In 1955 scientists were able to produce the tremen- dous pressure (over 100,000 times normal) and temperatures over 2,500°C to form a synthetic diamond from graphite that appears to be as real as a naturally formed diamond. However,
193 these high-grade “real” man-made diamonds are much too expensive to mass-produce.
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Nevertheless, the natural supply of diamonds cannot meet the needs of industry, so low-grade industrial diamonds are produced synthetically by forcing graphite, under great pressure and temperature, to form industrial diamonds that are used as abrasives and are placed on the tips of saw blades to improve wearability.
Today, carbon chemistry is more closely related to organic and hydrocarbon chemistry than to the elemental allotropes of carbon. Over the past century organic and hydrocarbon chemistry has opened up vast areas of research and development leading to new commercial processes and products.
CommonUses There are many uses for the very versatile element carbon. It, no doubt, forms more
compounds than any other element, particularly in the world of modern carbon chemistry. Carbon’s nature allows the formation-rings and straight- and branched-chains types of com- pounds that are capable of adding hydrogen as well as many different types of elemental atoms to these structures. (See figure 5 in the book’s section titled “Atomic Structure” for a depic- tion of a snake eating its tail as an analogy for the carbon ring of benzene.) In addition, these ringed, straight, and branched carbon molecules can be repeated over and over to form very large molecules such as the polymers, proteins, and carbohydrates that are required for life.
Carbon is an excellent reducing agent because it readily combines with oxygen to form CO
and CO 2 . Thus, in the form of coke in blast furnaces, it purifies metals by removing the oxides and other impurities from iron. Carbon, as graphite, has strong electrical conductivity properties. It is an important component in electrodes used in a variety of devices, including flashlight cells (batteries). Amorphous carbon has some superconduction capabilities.
Graphite is used for the “lead” in pencils, as a dry lubricant, and as electrodes in arc lamps. Of course, carbon is a popular jewelry item (e.g., diamonds). Future uses of carbon in the forms of fullerenes (C 60 up to C 240 ) and applications of nano- technology will provide many new and improved products with unusual properties. (See the section on “Atomic Structure” for more on these topics.)
ExamplesofCompounds Carbon-14 is a naturally occurring radioactive form of carbon with a half-life of 5,730 years.
Carbon dating is used to “date” any type of substance that was at one time “living.” A small amount of C-14 is always found with C-12. Because carbon-14 is radioactive, the rate of decay of carbon-14 can be calculated accurately to confirm the date when the organic substance was living. (Carbon was not “lost” over time, nor is any more added to the ancient sample, but C-14 does lose a specific amount of radiation at a given rate over time.) Therefore, calculating the ratio between C-12 and C-14 in an organic sample will tell you when that sample was alive.
Sucrose (C 12 H 22 O 11 ) is one of many forms of sugars (carbohydrates) that are important organic compounds for maintaining life. Carbon dioxide (CO 2 ) is the 18th most frequently produced chemical in the United States. It has numerous uses, including in refrigeration, in the manufacture of carbonated drinks (e.g., soda pop), in fire extinguishers, in providing an inert atmosphere (unreactive environ- ment), and as a moderator for some types of nuclear reactors.
194 | The History and Use of Our Earth’s Chemical Elements
Hydrocarbons are used as fuels and as the basic source of many other chemical compounds. The production of coke from coal also produces by-products known as coal-tars, which are used in the pharmaceutical, dye, food, and other industries. The refining of crude oil produces gasoline and many other fractions of the crude oil as well as petrochemical by-products. The range of useful products we derive from crude oil is very broad. These products not only power our automobiles, trucks, trains, and planes, but also provide the base for many of our medicines, foods, and numerous other essential products. (See the section of the book titled “Atomic Structure” for more on the chemistry of hydrocarbons.)
Hazards Many compounds of carbon, particularly the hydrocarbons, are not only toxic but also
carcinogenic (cancer-causing), but the elemental forms of carbon, such as diamonds and graphite, are not considered toxic.
Carbon dioxide (CO 2 ) in its pure form will suffocate you by preventing oxygen from enter- ing your lungs. Carbon monoxide (CO) is deadly, even in small amounts; once breathed into the lungs, it replaces the oxygen in the bloodstream.
Carbon dioxide is the fourth most abundant gas in the atmosphere at sea level. Excess CO 2 produced by industrialized nations is blamed for a slight increase in current temperatures around the globe. CO 2 makes up only 0.03+ percent by volume of the gases in the atmo- sphere. However, even a small amount in the upper atmosphere seems to be responsible for some global warming. Since pre-industrial times, the concentration of CO 2 in the Earth’s atmosphere has risen by approximately one-third, from 280 ppm (parts per million) to about 378 ppm. At the same time methane (CH 4 ) doubled its concentration over the years to about
2 ppm in the atmosphere. Methane is many times more effective as a “greenhouse” gas than is carbon dioxide, even though it breaks down in a shorter period of time. Some Scandinavian countries have experimented with pumping excess CO 2 produced by their industries deep onto the ocean floor where it will reenter the carbon cycle just as it does through trees and vegetation on the surface of the Earth. There are a number of super-computer programs attempting to predict the extent of global warming. The problem is the number of variables affecting climate change. The process is akin to trying to determine the shape of a cloud over the next hour. Unfortunately, neither well-meaning politicians nor scientists can agree on the extent of potential damage that excess carbon dioxide may do to the Earth in the future. Global warming and cooling are cyclic, which means that these processes have been alternat- ing over eons of time.
SILICON SYMBOL:Si PERIOD:3 GROUP:14(IVA) ATOMICNO:14
ATOMICMASS:28.0855amu VALENCE:4 OXIDATIONSTATES:+2,+4and–4 NATURALSTATE:Solid ORIGINOFNAME:SiliconwasnamedaftertheLatinwordsilex,whichmeans“flint.” ISOTOPES:Thereare21isotopesofsilicon,threeofwhicharestable.TheisotopeSi-28
makesup92.23%oftheelement’snaturalabundanceintheEarth’scrust,Si-29con- stitutes4.683%ofallsiliconfoundinnature,andthenaturalabundanceofSi-30is merely3.087%ofthestablesiliconisotopesfoundintheEarth’scrust.
Guide to the Elements |
ELECTRONCONFIGURATION EnergyLevels/Shells/Electrons Orbitals/Electrons
s2,p6
3-M=4
s2,p2
Properties Silicon does not occur free in nature, but is found in most rocks, sand, and clay. Silicon
is electropositive, so it acts like a metalloid or semiconductor. In some ways silicon resembles metals as well as nonmetals. In some special compounds called polymers, silicon will act in conjunction with oxygen. In these special cases it is acting like a nonmetal.
There are two allotropes of silicon. One is a powdery brown amorphous substance best known as sand (silicon dioxide). The other allotrope is crystalline with a metallic grayish luster best known as a semiconductor in the electronics industry. Individual crystals of sili- con are grown through a method known as the Czochralski process. The crystallized silicon is enhanced by “doping” the crystals (adding some impurities) with other elements such as boron, gallium, germanium, phosphorus, or arsenic, making them particularly useful in the manufacture of solid-state microchips in electronic devices.
The melting point of silicon is 1,420°C, its boiling point is 3,265°C, and its density is
2.33 g/cm 3 . Characteristics The characteristics of silicon in some ways resemble those of the element germanium,
which is located just below it in the carbon group. Flint is the noncrystalline form of silicon and has been known to humans since prehistoric times. When struck with a sharp blow, flint would flake off sharp-edged chips that were then used as cutting tools and weapons.
In addition to silica (silicon dioxide SiO 2 ), the crystal form of silicon is found in several semiprecious gemstones, including amethyst, opal, agate, and jasper, as well as quartz of vary- ing colors. A characteristic of quartz is its piezoelectric effect. This effect occurs when the quartz crystal is compressed, producing a weak electrical charge. Just the opposite occurs when electric vibrations are fed to the crystal. These vibrations are then duplicated in the crystal. Quartz crystals are excellent timekeeping devices because of this particular characteristic.
AbundanceandSource Silicon, in the form of silicon dioxide (SiO 2 ), is the most abundant compound in the
Earth’s crust. As an element, silicon is second to oxygen in its concentration on Earth, yet it is
196 | The History and Use of Our Earth’s Chemical Elements only the seventh most abundant in the entire universe. Even so, silicon is used as the standard
(Si = 1) to estimate the abundances of all other elements in the universe. For example, hydro- gen equals 40,000 times the amount of silicon in the cosmos. Hydrogen is the most abundant of all elements in the universe, and carbon is just three and half times as abundant as silicon in the entire universe. On Earth silicon accounts for 28% of the crust, oxygen makes up 47% of the crust, and much of the rest of the crust is composed of aluminum.
It is believed that silicon is the product of the cosmic nuclear reaction in which alpha par- ticles were absorbed at a temperature of 10 9 Kelvin into the nuclei of carbon-12, oxygen-16, and neon-20. Pure elemental silicon is much too reactive to be found free in nature, but it does form many compounds on Earth, mainly oxides as crystals (quartz, cristobalite, and tridymite) and amorphous minerals (agate, opal, and chalcedony). Elemental silicon is produced by
reducing silica (SiO 2 ) in a high-temperature electric furnace, using coke as the reducing agent. It is then refined. Silicon crystals used in electronic devices are “grown” by removing starter crystals from a batch of melted silicon.
History The practice of heating and forming glass-like figures from melted silica (sand) was known
at least three or four thousand years ago. In the eighteenth century early chemists were aware of some kind of link between sand (silica) and quartz but were unaware that a new, unidenti- fied element was involved.
Sir Humphry Davy attempted to isolate this unidentified element through electroly- sis—but failed. It was not until 1824 that Jöns Jakob Berzelius (1779–1848), who had earlier discovered cerium, osmium, and iridium, became the first person to separate the element sili- con from its compound molecule and then identify it as a new element. Berzelius did this by
a two-step process that basically involved heating potassium metal chips with a form of silica (SiF 4 = silicon tetrafluoride) and then separating the resulting mixture of potassium fluoride and silica (SiF 4 + 4K → 4KF + Si). Today, commercial production of silicon features a chemi- cal reaction (reduction) between sand (SiO 2 ) and carbon at temperatures over 2,200°C (SiO 2 + 2C + heat→ 2CO + Si). Silicon research carried out in the nineteenth and early twentieth centuries led to many forms and uses of silicon and its compounds, including silicone plastics, resins, greases, and
polymers.
CommonUses Silicon’s tetravalent pyramid crystalline structure, similar to tetravalent carbon, results in
a great variety of compounds with many practical uses. Crystals of silicon that have been contaminated with impurities (arsenic or boron) are used as semiconductors in the computer and electronics industries. Silicon semiconductors made possible the invention of transistors at the Bell Labs in 1947. Transistors use layers of crystals that regulate the flow of electric cur- rent. Over the past half-century, transistors have replaced the vacuum tubes in radios, TVs, and other electronic equipment that reduces both the devices’ size and the heat produced by the electronic devices.
Silicon can be used to make solar cells to provide electricity for light-activated calculators and satellites. It also has the ability to convert sunlight into electricity.
197 When mixed with sodium carbonate (soda ash) and calcium carbonate (powdered lime-
Guide to the Elements |
stone) and heated until the mixture melts, silica (sand) forms glass when cooled. Glass of all types has near limitless uses. One example is Pyrex, which is a special heat-resistant glass that is manufactured by adding boron oxide to the standard mixture of silica, soda ash, and limestone. Special glass used to make eyewear adds potassium oxide to the above standard mixture.
Silicon is also useful as an alloy when mixed with iron, steel, copper, aluminum, and bronze. When combined with steel, it makes excellent springs for all types of uses, including automobiles.
When silicon is mixed with some organic compounds, long molecular chains known as silicone polymers are formed. By altering the types of organic substances to these long silicone polymer molecules, a great variety of substances can be manufactured with varied physical properties. Silicones are produced in liquid, semisolid, and solid forms. Silicones may be rubbery, elastic, slippery, soft, hard, or gel-like. Silicone in its various forms has many com- mercial and industrial uses. Some examples are surgical/reconstructive implants, toys, Silly Putty, lubricants, coatings, water repellents for clothing, adhesives, cosmetics, waxes, sealants, and electrical insulation.
ExamplesofCompounds Silicon dioxide (SiO 2 ) is the most abundant compound in the Earth’s crust. Known as com-
mon sand, it also exists in the forms of quartz, rock crystal, amethyst, agate, flint, jasper, and opal. It has many industrial uses.
Sodium silicate (Na 2 SiO 3 ), better known as water glass, is one of the few silicon compounds that dissolves in water. Produced at high temperatures (SiO 2 + 2NaOH + heat→ H 2 O + Na 2 SiO 3) , it is used in the manufacture of soaps, adhesives, and food preservatives. Silicon nitride (Si 3 N 4 ), resistant to oxidation, is an excellent coating for metals, as well as an adhesive and abrasive, and it is used in high-temperature crucibles. It has proven useful as heat-resistant substance for the nozzles on rocket engines.