CHEM-I-PAGE

Silver                                                                                                                                        

Standard Sate: solid at 298 K

Color: silver

Silver is somewhat rare and expensive, although not as expensive as gold. Slag dumps in Asia Minor and on islands in the Aegean Sea indicate that man learned to separate silver from lead as early as 3000 B.C. Pure silver has a brilliant white metallic lustre. It is a little harder than gold and is very ductile and malleable. Pure silver has the highest electrical and thermal conductivity of all metals, and possesses the lowest contact resistance.

(Si), a nonmetallic chemical element in the carbon family (Group IVa
of the periodic table). Silicon makes up 27.7 percent of the Earth's
crust; it is the second most abundant element in the crust, being
surpassed only by oxygen.

Silicon was first isolated and described as an element in 1824 by J�ns
Jacob Berzelius, a Swedish chemist. Silicon does not occur
uncombined in nature; but it is found in practically all rocks as well as
in sand, clays, and soils, combined either with oxygen as silica (SiO2,
silicon dioxide) or with oxygen and other elements (e.g., aluminum,
magnesium, calcium, sodium, potassium, or iron) as silicates. Its
compounds also occur in all natural waters, in the atmosphere (as
siliceous dust), in many plants, and in the skeletons, tissues, and body
fluids of some animals.

Pure silicon is a hard, dark gray solid with a metallic lustre and with
a crystalline structure the same as that of the diamond form of
carbon, to which silicon shows many chemical and physical
similarities. A brown, powdery form of silicon has been described
that also has a microcrystalline structure. The element is prepared
commercially by reducing (removing the oxygen from) the oxide by its
reaction with coke in electric furnaces. On a small scale, silicon can
be obtained from the oxide by reduction with aluminum.

Silicon, like carbon, is relatively inactive at ordinary temperatures; but
when heated it reacts vigorously with the halogens (fluorine, chlorine,
bromine, and iodine) to form halides and with certain metals to form
silicides. It is unaffected by acids except hydrofluoric. At red heat,
silicon is attacked by water vapour or by oxygen, forming a surface
layer of silicon dioxide. When silicon and carbon are combined at
electric furnace temperatures (2,000-2,600 C [3,600-4,700 F]), they
form silicon carbide (Carborundum, SiC), which is an important
abrasive. In union with hydrogen, silicon forms a series of hydrides,
the silanes. When united with hydrocarbon groups, silicon forms a
series of organic silicon compounds.

Silicon's atomic structure makes it an extremely important
semiconductor; highly purified silicon, doped with such elements as
boron, phosphorus, and arsenic, is the basic material used in computer
chips, transistors, silicon diodes, and various other electronic circuits
and switching devices. Silicon of lesser purity is used in metallurgy as
a reducing agent and as an alloying element in steel, brass, and
bronze. The most important compounds of silicon are the dioxide
(silica) and the various silicates. Silica in the form of sand and clay is
used to make concrete and bricks as well as refractory materials for
high-temperature applications. As the mineral quartz, the compound
may be softened by heating and shaped into glassware. Silicates, most
of which are insoluble in water, are employed in making glass as well
as in the fabrication of enamels, pottery, china, and other ceramic
materials. Sodium silicates, commonly known as water glass, or
silicate of soda, are used in soaps, in the treatment of wood to prevent
decay, for the preservation of eggs, as a cement, and in dyeing.
Silicones are synthetic organosilicon oxides composed of the elements
silicon, oxygen, carbon, and hydrogen; they are used as lubricants,
hydraulic fluids, waterproofing compounds, varnishes, and enamels
because, as a class, they are chemically inert and unusually stable at
high temperatures.

Three stable isotopes of silicon are known: silicon-28, which makes
up 92.21 percent of the element in nature; silicon-29, 4.70 percent;
and silicon-30, 3.09 percent. Five radioactive isotopes are known.
atomic number 14 atomic weight 28.086 melting point 1,410 C (2,570
F) boiling point 2,355 C (4,270 F) density 2.33 g/cm3 oxidation state
-4, (+2), +4 electron config. 2-8-4 or 1s22s22p63s23p2

"silicon" Encyclop�dia Britannica Online.

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