Group 4A (or IVA) of the periodic table includes the nonmetal carbon (C), the metalloids silicon (Si) and germanium (Ge), the metals tin (Sn) and lead (Pb), and the yet-unnamed artificially-produced element ununquadium (Uuq).
The Group 4A elements have four valence electrons in their highest-energy orbitals (ns2np2). Carbon and silicon can form ionic compounds by gaining four electrons, forming the carbide anion (C4-) and silicide anion (Si4-), but they more frequently form compounds through covalent bonding. Tin and lead can lose either their outermost p electrons to form 2+ charges (Sn2+, the stannous ion, and Pb2+, the plumbous ion) or their outermost s and p electrons to form 4+ charges (Sn4+, the stannic ion, and Pb4+, the plumbic ion).
Carbon (C, Z=6).
Carbon is most familiar as a black solid is graphite, coal, and charcoal, or as the hard, crystalline diamond form. The name is derived from the Latin word for charcoal, carbo. It is found in the Earth's crust at a concentration of 480 ppm, making it the 15th most abundant element. It is found in form of calcium carbonate, CaCO3, in minerals such as limestone, marble, and dolomite (a mixture of calcium and magnesium carbonate); calcium carbonate also forms the shells of marine organisms and the coral of coral reefs. Carbon is also found in coal, petroleum, and natural gas.
Carbon is one of the most important elements on the periodic table (at least from the perspective of organic chemists!). Pure carbon is found in three stable forms at room temperature: graphite, diamond, and the fullerene form. In graphite, the carbon atoms are connected in sheets, which can slide past each other, which makes graphite able to act as a lubricant, and why it makes marks on paper in the form of pencil "lead." In diamonds, the carbon atoms are held together by covalent bonds in a rigid, three-dimensional framework, which results in a extremely hard and rigid structure. (Contrary to the James Bond title, diamonds aren't forever, since the graphite form is more stable; however, diamonds turn into graphite at an incredibly slow rate.) Diamonds are so different in their physical characteristics from graphite or charcoal that it was not recognized the diamond was a form of carbons; this was shown by Antoine Lavoisier in 1784 when he demonstrated that both charcoal and diamond could be burned to produce carbon dioxide. Smithson Tennant confirmed this relationship in 1796 when he showed that equal amounts of charcoal and diamond produced equal amounts of carbon dioxide. In the fullerene form, the carbon atoms are arranged in hollow balls, or in hollow tubes (called "nanotubes"); these forms of carbon have very interesting chemical physical properties, and are the subject of intense research by chemists and chemical engineers.
Carbon is produced in stars by the triple alpha process, in which three alpha particles are converted into carbon-12. In this process, two alpha particles (helium nuclei, 42He) fuse to form beryllium-8, which then fuses with another alpha particle to produce carbon-12:
42He + 42He ® 84Be
84Be + 42He ® 126C + g
This process takes place in older stars where a lot of hydrogen has been converted into helium; the star collapses, raising the pressure and temperature in the core to above 100 million Kelvins, initiating the process of helium burning.
Some ionic compounds of carbon are known, but carbon typically forms compounds through covalent bonding. Carbon forms strong, stable covalent bonds to other carbon atoms, and is capable of forming long chains containing anywhere from a few dozen carbon atoms to hundred of thousands of carbon atoms. Carbon can also form bonds to other elements, such as hydrogen, oxygen, nitrogen, sulfur, phosphorus, the halogens, etc. There are therefore a tremendous variety of complex carbon-based chemicals. Organic chemistry is the field of chemistry concerned with the study of carbon-containing compounds. Such compounds form the basis of life (at least the kinds that we know about).
Carbon is found is coal, and petroleum is a very complex mixture of thousands of different hydrocarbons. The burning of carbon and petroleum products (fossil fuels) provides most of the energy which we consume, and contributes to global warming through the release of carbon dioxide into the atmosphere. Carbon is used is the refining of iron and other metals (the oxygen in the ores is carried away in the form of carbon dioxide, leaving behind the elemental metal). Small amounts of carbon are added to iron to make an alloy called steel, which is harder than pure iron. Activated charcoal is a finely powdered form of carbon used to filter out impurities from water or gases.
Carbon is taken up by green plants in the form of carbon dioxide, CO2; in the process of photosynthesis, the carbon in the carbon dioxide is transformed into carbohydrates (sugars), lipids, proteins, and all of the other organic molecules which are essential to life.
Most carbon is in the form of the carbon-12 isotope (98.90%), which has 6 protons and 6 neutrons in its nucleus. Carbon-13, which is also non-radioactive, accounts for 1.10% of the world's carbon. Carbon-13 is particularly important in nuclear magnetic resonance spectroscopy (see the section on Hydrogen); organic molecules contain small amounts of carbon-13, which responds to magnetic fields in a similar fashion as hydrogen-1. Carbon-14, which consists of 6 protons and 8 neutrons, is an unstable isotope produced the reaction of free neutrons (produced from cosmic rays) with nitrogen-14 in the upper atmosphere. Carbon-14 undergoes beta decay to produce nitrogen-14, with a half-life of 5730 years:
147N + 10n ® 146C + 11H
146C ® 147N + 0-1b
The amount of carbon-14 thus produced is extremely small — approximately 7 kilograms per year — but small amounts of this carbon-14 are taken up in the form of carbon dioxide along with the "normal" isotopes of carbon by green plants, and this isotope also becomes incorporated into the things that eat the green plants (and also the things that eat the things that eat the green plants — and so on). Once an organism dies, it stops taking in carbon-14 (or anything else, for that matter), and the carbon-14 that it had at the moment of death decays, and is no longer replaced. By measuring the amount of carbon-14 remaining in an organic sample, it is possible to determine how long ago the organism died. This technique works for carbon-containing materials that are up to about 50,000 years old; beyond that, there is too little carbon-14 remaining to get an accurate date, and some other form of radiometric dating must be used. This technique was developed by Willard F. Libby in the 1950s, who received the Nobel Prize in Chemistry in 1960 for this work.
Silicon (Si, Z=14).
Silicon is a dark gray element with a metallic luster. The name of the element is derived from the Latin word for flint, silicis. It is found in the Earth's crust at a concentration of 28%, making it the second most abundant element. In the form of silica (SiO2) or one of the silicates (SiO44-), it is found in many different minerals, including clay, quartz, zircon, feldspar, mica, zeolites, aluminosilicates, sand, etc. It is also found in the gemstones opal, agate, rhinestone, and amethyst.
Silicon is one of the most important elements on the periodic table (at least from the perspective of computers!). Ultrapure silicon doped with boron or phosphorus is used as semiconductors in transistors, which are heavily employed in computers, solar panels, and other applications. Silica (which is primarily silicon dioxide, SiO2), is used in the manufacture of glass. Silicones, which consist of chains of alternating silicon and oxygen atoms, are used in oils, lubricants, and silicone rubber.
Germanium (Ge, Z=32).
Germanium is a hard, grayish white element with a metallic luster. The name of the element is derived from the Latin word for Germany, Germania. It is found in the Earth's crust at a concentration of 2 ppm, making it the 52nd most abundant element. It is found in the ores argyrodite [Ag8GeS6] and germanite [Cu13Fe2Ge2S16], but is more frequently obtained as a by-product of the refining of zinc.
Like silicon, germanium is used as a semiconductor, and is widely used in the computer industry. Silicon and germanium are both metalloids, having some characteristics of both metals and nonmetals.
The existence of germanium was predicted by Dimitri Mendeleev in 1869 from a blank space in his periodic table beneath silicon; before it was actually found, the hypothetical element was referred to as "eka-silicon." When germanium was discovered in 1886, its physical and chemical properties matched many of those predicted by Mendeleev.
Tin (Sn, Z=50).
Tin is a soft, silvery-white metal. The name of the element is derived from the Anglo-Saxon word for the metal, while the chemical symbol "Sn" is derived from the Latin name for the metal, stannum. It is found in the Earth's crust at a concentration of 2 ppm, making it the 49th most abundant element. It is found in the ore cassiterite [tin(IV) oxide, SnO2], and in trace amounts in other minerals.
Elemental tin exists in two allotropic forms: above 13.2 °C, it is found as white or beta tin, which is the crystalline, metallic form; below 13.2°C it is found as gray or alpha tin, which has a powdery appearance. Structures made of tin that are cooled below 13.2°C can start to crumble, a condition known as "tin pest" or "tin disease." This can be prevented by alloying the tin with a small amount of antimony or bismuth.
Tin is easily purified in its metallic form from its ores, and has been known since prehistoric times. Tin is commonly plated onto iron, forming a protective surface that prevents the iron from rusting; this is extremely useful in food containers (tin cans) because the tin is nontoxic and is not corrosive. Tin can be hammered into thin sheets ("tin foil"), but this application has been replaced with aluminum foil. Tin is also used in alloys such as bronze (95% copper and 5% tin), solder (33% tin and 67% lead), pewter (85% tin, 7% copper, 7% antimony, and 4% copper), and dental amalgams (60% silver, 27% tin, and 13% copper). Bronze has been used since at least 3000 BC, since it is harder than copper and more easily made into tools, weapons, works of art, etc.
Lead (Pb, Z=82).
Lead is a very dense, soft, very malleable, bluish-white or grayish metal. The name of the element is derived from the Anglo-Saxon word for the metal, while the chemical symbol "Pb" is derived from the Latin name for the metal, plumbum. It is found in the Earth's crust at a concentration of 14 ppm, making it the 36th most abundant element. It is found in the ores galena [lead(II) sulfide, PbS], cerussite [lead(II) carbonate, PbCO3], anglesite [lead(II) sulfate, PbSO4], pyromorphite [Pb5(PO4)3Cl], and boulangerite [Pb5Sb4S11].
Despite its relative rarity, lead has been known since ancient times, since it is comparatively easy to refine and purify. Lead used to be used in plumbing, since lead pipes don't corrode the way that iron pipes do; the Latin name for lead, plumbum, is also the root of the words "plumbing" and "plumber." It is also used in paints, solders, batteries (such as the lead-acid storage batteries found in cars), and radiation shielding. Lead is the end product of the radioactive decay of many heavier elements; the ratio of other elements to lead can be used in radioactive dating of rocks.
Lead used to be used to sweeten wine, through the formation of lead(IV) acetate, Pb(C2H3O2)4, also known as "sugar of lead." Lead was also used as an octane booster for gasoline, in the form of tetraethyl lead, Pb(CH2CH3)4, but this has been phased out due to environmental concerns. The earliest pencils used lead, although now "pencil lead" is graphite mixed with clay.
Lead is a cumulative poison; absorption of lead in the body over a long period of time causes it to be stored in the bones in the form of lead phosphate, where it interferes with the production of hemoglobin, leading to anemia, stomach cramps, constipation, headaches, infertility, etc.
Ununquadium (Uuq, Z=82).
Ununquadium is a synthetic element, produced by the fusion of an isotope of plutonium with an isotope of calcium. "Ununquadium" is a temporary, systematic name (literally meaning "1" "1" "4", the atomic number of the element) until the official name is decided upon. The longest-lived isotope produced so far, ununquadium-289, has a half-life of 30.4 seconds. It was first produced at the Joint Institute for Nuclear Research (JINR) at Dubna in 1998. Other isotopes have been produced in other labs, with a wide range of half-lives. It has been predicted that an isotope having 114 protons and 184 neutrons, ununquadium-298, should be especially stable, but this isotope has yet to be produced.
John Emsley, The Elements, 3rd edition. Oxford: Clarendon Press, 1998.
John Emsley, Nature's Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2001.
David L. Heiserman, Exploring Chemical Elements and their Compounds. New York: TAB Books, 1992.