Group 14: The Carbon Family
Carbon is one of the most common elements on Earth, and indeed it is common is our everyday life. It is all around us. Perhaps you have heard of Carbon Dioxide (CO2) or Methane (CH4), the most common molecules containing carbon. All kinds of scientists study carbon -- biologists investigating the origins of life, oceanographers measuring the acidification of the oceans, and engineers developing diamond films for their tools, to name just a few. This article, though, will focus on the periodic properties of the carbon family and include brief discussions of the individual properties of carbon, silicon, tin, lead, germanium, and ununquadium.
The carbon family, group 14 in the p block, contains carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), and Flerovirum (Fl). These elements have only two electrons in the outermost p orbital. That is, their valence electron configuration is ns2np2. They tend to have oxidation states of +4 and, for the heavier elements, +2 due to the inert pair effect.
Figure 1: Row 14 in the Periodic Table
Members of this group conform well to general periodic trends. The atomic radii increase as you move down the group, and ionization energies decrease. Metallic properties increase as you move down the group. Carbon is a non-metal; silicon and germanium are metalloids; and tin and lead are poor metals. (For example, they do not conduct heat and electricity as well as, say, copper.)
Despite their adherence to periodic trends, the properties of the Carbon family vary greatly. Carbon is a non-metal. Tin and Lead behave entirely as metals. In their elemental solid state, Group 14 metalloids, silicon and germanium, act as electrical semiconductors, although silicon is mainly non-metallic. Their electrical conductivity can be affected in various degrees by doping - the addition of Group 13 or Group 15 elements in various concentrations to the Group 14 solid matrix. These semiconductor properties have wide application for circuitry components in the electronics industry, such as diodes, transistors, and integrated circuit (IC) chips.
Carbon is the fourth most abundant element on earth. Carbon is a very special element. It's what makes organic molecules organic. It is the "backbone" of biology. Why is this? Why don't we have life based on silicon, or some other element, like iron? Carbon has two important qualities: small size and a unique electron configuration. Since it's small, the p-orbital electrons overlap considerably and enable pi bonds to form. (For instance, compare the molecular structures of CO2 and SiO2 below.)
CO2 has double bonds between carbon and oxygen, whereas SiO2 has single bonds. Note: The CO2 molecule can exist on its own, in the gas phase. The SiO2 molecule, on the other hand, would always be within a network of covalent bonds.
The electron configuration of Carbon allows it to form very stable bonds with oxygen and hydrogen. These bonds can store an enormous amount of energy. Formation and combustion of these bonds (the carbon cycle) drives life on earth.
6CO2 + 6H2O + energy → C6H12O6 + 6O2
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
Carbon is often referenced for its allotropes. It is second next to sulfur as the element with the most allotropes. Carbon has three main solid state allotropes: graphite, diamond and fullerenes (or fullerenes' more memorable name: Buckyballs). These allotropes differ greatly in form but are widely used in modern production.
Graphite has lubricating properties that make it extremely suitable and now ubiquitous in pencils. Because it is arranged in planes that can easily slide pass one another, graphite glides easily and is hence used in combination with clay to form pencil "lead." Graphite is also used in a fiber form for various plastics.
Carbon has very high melting and boiling points. Graphite is the most thermodynamically stable allotrope of carbon. In diamonds, the more stable form at extreme pressures (105 atm and up), each carbon atom is bonded to four others in a tetrahedral arrangement, resulting in the hardest naturally-occurring substance known. This hardness, combined with a good ability to dissipate heat, makes diamonds and diamond films an excellent material in drill bits and other machine parts; however, the highest-quality natural diamonds are used mainly for jewelry, while lower-grade diamond or even synthetic diamond is used for industrial purposes.
Fullerenes (after R. Buckminster Fuller) and nanotubes are a series of carbon allotropes in which carbon rings form more complex forms -- soccerball-like molecules (C60) or tubes that resemble a cylinder made of chicken wire. Graphene, a carbon sheet made of single carbon atoms with intriguing electronic properties, is the basis for all of these allotropes. Fullerenes occur when a certain percentage of hexagonal rings are reassembled into pentagonal rings, causing the sheet to contort into a roughly spherical "Buckyball." A carbon nanotube is simply graphene bent into itself to form a cylinder. Some of these allotropes come from the decomposition of graphite. Combustion can also yield alternate carbon forms. Heated coal without air becomes coke. Similarly heated wood becomes charcoal as more volatile integrands are forced away.
Graphite and a diamond. (Libby didn't have any nanotubes lying around the house.)
Jessica's C60, a "buckyball". Press play to see it rotate, or pause and scroll through for slower rotation.
A nanotube model.
There are a nearly innumerable amount of different carbon compounds, but several inorganic compounds are particularly important. Carbon monoxide (CO) is used for synthesizing further compounds, reducing metal compounds to usable products, and in combination with other gases for fuel. Carbides, compounds of carbon and metals, are used for many markets, often to stabilize other metal structures. Calcium carbide is used to fabricate industrial chemical compounds. Carbon disulfide and carbon tetrachloride are powerful solvents, (although since being declared a carcinogen, CCl4 use has declined). Cyanide acts similarly to halide ions, forming both a salt and an acid. Hydrocyanic acid (HCN) is a weak acid with extremely low boiling point (room temperature in fact), and is used in plastic production. Cyanides' two forms together form cyanogen, which is used in organic syntheses, fumigants, and rocket propellant.
Although silicon plays a much smaller role in biology, it still plays an important role in our world. It is the second most common element in the earth's crust (after oxygen) and is the backbone of the mineral world. It is neither a metal or nonmetal, but a metalloid. Silicon is an inert metal, mainly reacting with halogens. It may have acted as a catalyst in the formation of the earliest organic molecules (Sadava 62). Plants depend on silicates (such as [SiO4]4-) to hold nutrients in the soil, where their roots can absorb them (Sadava 787). People around the world have been using silicon (primarily in the silica SiO2 molecule) for millenia in the creation of ceramics and glass. In more recent history, the name "Silicon Valley" attests to its importance in the computing industry-if carbon is the backbone of human intelligence, silicon is the backbone of artificial intelligence. Silicon is found in beach sand, and is useful in making concrete and brick.
Tin is a soft, malleable metals with a low melting point. It has two solid-state allotropes at regular temperatures and pressures, denoted by the Greek 'alpha' and 'beta' letters. At higher temperatures (above 13°C), tin exists as white tin , or beta-tin, and is often used in alloys. At lower temperatures, tin can transform into grey tin, (alpha-tin); it loses its metallic properties and turns powdery. This could cause the disintegration of items made from white tin alloys that have been exposed to the cold for long periods of time. The pipes in Europe's great pipe organs are a classic victim of this "tin pest." When a crystalline is broken, a "tin cry" is heard; this happens when a bar is bent. Most of us probably encounter tin (grey tin) every day in our kitchens, where we find it preserving our canned goods; the cans are iron but are plated with tin to prevent them from rusting. Tin is malleable, ductile, and a crystalline. It has 27 isotopes-9 that are stable and 18 that are not stable. It is a superconductor at low temperatures. Tin reacts with alkalis, acid salts, and strong acids. The chlorides of tin are good reducing agents and often used to reduce iron ores. Tin fluoride is often the anticavity "fluoride" additive in toothpastes.
Lead, aka plumbate, is similar to tin in that it is also a soft, malleable metal with a low melting point. It is used to be common in water and sewage pipes-hence the terms plumber and plumbing. Lead is toxic to humans, especially children. Even low levels of exposure can cause nervous system damage and can prevent proper production of hemoglobin (the molecule in red blood cells responsible for bringing oxygen to your body). Because of this, there has been a concerted effort to reduce people's exposure to lead - we now buy unleaded gasoline and unleaded paint. Wikipedia has a neat article on the history and physiology of lead poisoning. Lead usually has an oxidation state of +2 or +4, especially when it is stable. Its oxides have many industrial uses as oxidizing agents. One demonstrative example is as a cathode in lead-acid storage cells.
Germanium is a rare element that is used in the manufacture of semi-conductor devices. The physical and chemical properties of germanium are very similar to that of Silicon. The semi-metal is found in coal, ore, and germanite. The color is gray-white crystalline.
Flerovium (Fl) is also known as Element 114. It was found in 1998 by scientists in Dubna. It is radioactive.
Here are some questions to test your understanding of this material. To see the answers, highlight the white text with your mouse.
Ans: They do not conduct heat or electricity very well.
Ans: It is composed of flat sheets, which are weakly bonded to one another, so they easily slide past each other and rub off on your paper.
Ans: Each carbon atom forms bonds with four other carbon atoms in a tetrahedral crystal. This arrangement is extremely strong.
Ans: The tin plating prevents the iron can from oxidizing (rusting).
Ans: Since the valence electron configuration is ns2np2, the atoms tend to lose either all four outer shell electrons (resulting in a charge of +4) or, because of the inert pair effect, it may lose just the s electrons (resulting in a charge of +2).
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