CH226.16
The Carbon group (14 / IV)
Elements of the Carbon group (group 14/ IV)
C, carbon (the name is derived from carbo, the Latin for charcoal)
Si, silicon (the name is derived from the Latin silex or silics, meaning flint – a form of
quartz, SiO2)
Ge, germanium (the name is derived from Germania, the Latin name for Germany)
Sn, tin (the name is from the Anglo-Saxon tin – but it’s origin is unknown. The chemical
symbol Sn comes from the Latin stannum, which may mean hard)
Pb, lead (the name comes from the Anglo-Saxon word for the metal, lead. The
chemical symbol, Pb, derives from plumbum, the Latin name for the metal.)
Members of group IV have the electronic configuration ns2np2.
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Carbon
Carbon is found in nature in two forms (allotropes): graphite and diamond. More
recently, another form of carbon, fullerenes (of which buckminsterfullerene, C60, is the
best known), has been discovered. Carbon is present as CO2 in the atmosphere and
in all natural waters. It is a component of rocks as carbonates of calcium (limestone),
magnesium, and iron. Coal, petroleum, and natural gas are chiefly hydrocarbons.
Carbon is unique among the elements in the vast number of a variety of organic
compounds it can form. Carbon itself is non-toxic, but some simple inorganic
compounds can be very toxic, such as carbon monoxide (CO) or the cyanides (MCN−).
Graphite, one of the softest materials known, is primarily used as a lubricant. Diamond
is the hardest natural solid known. Naturally occurring diamond is typically used for
jewelry. For industrial purposes, diamonds are artificially produced by squeezing
graphite under high temperature and pressure for several days or weeks, and they are
primarily used to make instruments, like diamond tipped saw blades.
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Allotropes of carbon: diamond
Diamond has the highest melting point (~4000 °C, in
the absence of O2) of any element. The crystal lattice
of diamond consists of tetrahedral sp3 carbon atoms.
Two views of the crystal structure are shown below:
1.51 Å
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Allotropes of carbon: graphite
Graphite is thermodynamically marginally more stable
than diamond by 2.9 kJ/mol.
Graphite is slippery and soft and has a flaky
appearance. Graphite consists of planar layers of
hexagons formed by sp2 hybridized carbons. The
delocalization of electrons in the C-C π-bonds leads to
electrical conductivity.
The average C-C bond distance is 1.42 Å in graphite
(compare to C6H6: bond order = 1½ and the C-C
distance is 1.39 A).
Interlayer distance: 3.35 Å
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Allotropes of carbon: fullerenes
Fullerenes are isolated by extraction of specially prepared soot with organic
solvents. The most common fullerene is C60 (Buckminsterfullerene), which is a
molecule formed by sp2 carbons:
Fullerenes resemble geodesic domes designed by R. Buckminster Fuller:
(geodesic dome structures of the Eden Project in UK)
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Silicon
Silicon is not found free in nature. Crystalline silicon has a
metallic luster and grayish color. Silicon is a relatively inert
element, but it is attacked by halogens and dilute alkali. Most
acids do not affect it. Crystalline silicon, when doped with
elements such as boron, gallium, germanium, phosphorus or
arsenic, is used in the manufacture of solid-state electronic
devices, such as transistors, solar cells, rectifiers and
microchips. Silica (SiO2) is the most abundant material in the
Earth's crust. It commonly takes the form of ordinary sand, but
Amethyst is SiO2 colored
also exists as quartz and rock crystal.
by traces of manganese.
Silicon is one of man's most useful elements. In the form of sand and clay it is used to
make concrete and brick; it is a useful refractory material for high-temperature work,
and in the form of silicates it is used in making enamels, pottery, etc. Silica, as sand, is
a principal ingredient of glass, one of the most inexpensive of materials with excellent
mechanical, optical, and thermal properties.
Silicon is non-toxic as the element and in most natural forms. Only some silicates with
a fibrous structure, such as asbestos (a magnesium silicate), are carcinogenic and can
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cause lung cancer.
Germanium
Ultra-pure germanium is silvery-white, brittle, semi-metallic
element (a metalloid) that is stable in air and water, and is
unaffected by alkalis and acids, except nitric acid.
The development of the germanium transistor opened the door to countless
applications of solid-state electronics. From 1950 through the early 1970s, this area
provided an increasing market for germanium, but then high purity silicon began
replacing germanium in transistors, diodes, and rectifiers. Meanwhile, demand for
germanium in fiber optics communication networks, infrared night vision systems, and
polymerization catalysts increased dramatically. These end uses represented 85% of
worldwide germanium consumption for 2000. The world production of germanium was
about 120 tonnes in 2010 (China is the main producer, 80 tonnes).
There is no biological role for germanium. There have been claims that germanium
could be beneficial for health, although this has not been proved scientifically.
Germanium compounds seem to be effective in killing some types of bacteria and are
currently being studied for use in chemotherapy.
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Tin
Two allotropes of tin occur near room temperature. The first
form of tin is called gray tin and is stable at temperatures
below 13.2 °C. There are few, if any, uses for gray tin. At
temperatures above 13.2 °C, gray tin slowly turns into white
tin. This process is reversible - an effect known as ‘tin plague’.
Tin plague has certainly resulted in the destruction of tin organ pipes in many
European cities during cold winters. When first noted, it was reportedly attributed to
the devil's work. Disintegration of tin dishes and cutlery in cold weather expeditions,
kerosene containers (Captain Robert Scott's Antarctic expedition) have been attributed
to this effect.
World production of Sn is around 140,000 tonnes per year. The metal itself is used in
alloys, for example, bronze is an alloy of tin and copper. Tin resists corrosion and is
used as a protective coating on other metals. A “tin can” is actually made from steel. A
thin layer of tin is applied to the inside and outside of the can to keep the steel from
rusting. Tin is used to form a number of useful compounds.
Inorganic tin compounds are generally regarded as non-toxic. However, organotin
compounds can be toxic. Trimethyltin and triethyltin are particularly toxic.
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Lead
Lead is a bluish-white lustrous metal. It is soft, highly
malleable, ductile, and a relatively poor conductor of
electricity. It is very resistant to corrosion but tarnishes
upon exposure to air.
Lead is used to line tanks that store corrosive liquids, such as sulfuric acid. Lead's
high density makes it useful as a shield against X-ray and gamma-ray radiation and it
is used in X-ray machines and nuclear reactors. Lead is also used as a covering on
some wires and cables to protect them from corrosion, as a material to absorb
vibrations and sounds and in the manufacture of ammunition. Most of the lead used
today is used in the production of lead-acid batteries, such as the batteries found in
automobiles. Tetraethyl lead (PbEt4) was widely used in gasoline as an anti-knock
agent, but had to be phased out on environmental and health grounds.
The ancient Romans used lead to make water pipes. Unfortunately for them, lead is
a cumulative poison and the decline of the Roman empire has been blamed, in part,
on lead in the water supply. Symptoms of mild lead poisoning (headaches, stomach
pains) can be easily overlooked. Prolonged exposure to lead eventually results in
anemia, because it deactivates the enzymes that make hemoglobin.
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