Silicon
Symbol |
Name |
Atomic Number |
Atomic Weight |
Group Number |
Si |
Silicon |
14 |
28.086 |
14 |
Standard Sate: solid at 298 K
Color: dark grey with a bluish tinge
Silicon is present in the sun and stars and is a principal component of a class of meteorites known as aerolites. Silicon makes up 25.7% of the earth's crust by weight, and is the second most abundant element, exceeded only by oxygen. It is found largely as silicon oxides such as sand (silica), quartz, rock crystal, amethyst, agate, flint, jasper and opal. Silicon is found also in minerals such as asbestos, feldspar, clay and mica.
(Se), a chemical element in the oxygen family (Group VIa) of the
periodic table, closely allied in chemical and physical properties with
the elements sulfur and tellurium.
Occurrence, characteristics, and uses.
Selenium was first recognized as an element in 1818 by J�ns Jacob
Berzelius, a Swedish chemist. It is a metalloid (an element
intermediate in properties between the metals and the nonmetals) that
is widely distributed throughout the world, but only in small quantities.
Selenium occasionally occurs uncombined, usually in conjunction
with free sulfur; it is more commonly found together with the sulfides
as the selenides in ores of such metals as iron, lead, silver, and
copper. When any of the selenium-containing sulfide minerals is
roasted, selenium appears as a by-product in the flue dusts. It is also
extracted from the anode slimes that remain after the electrolytic
refining of copper.
Selenium exists in several different forms, the three most important
being the amorphous (noncrystalline), which is red when in powder
form and black when in vitreous (glassy) form; the red crystalline;
and the gray metallic, which is also crystalline. Of the three, the
metallic form is the most stable under ordinary conditions; the other
forms very slowly convert to the metallic form at room temperature.
Because the electrical conductivity of metallic selenium increases
when light strikes it and because it can convert light directly into
electricity, the element is used in photoelectric cells, solar cells, and
photographic exposure meters. It is also used extensively in rectifiers
because of its ability to convert alternating electric current to direct
current. When incorporated in small amounts into glass, selenium
serves as a decolorizer; in larger quantities it imparts to glass a clear
red colour that is useful in signal lights. The element is also employed
in making red enamels for ceramics and steel ware, as well as for the
vulcanization of rubber to increase resistance to abrasion.
Compounds.
In its compounds selenium exists in the oxidation states of -2, +4,
and +6. It manifests a distinct tendency to form acids in the higher
valences. Although the element itself is not poisonous, many of its
compounds are exceedingly toxic.
Selenium combines directly with hydrogen, resulting in hydrogen
selenide, H2Se, a colourless, foul-smelling gas that is a cumulative
poison. It also forms selenides with most metals (e.g., aluminum
selenide, cadmium selenide, and sodium selenide).
In combination with oxygen, it occurs as selenium dioxide, SeO2, a
white, solid, chainlike polymeric substance that is an important reagent
in organic chemistry. The reaction of this oxide with water produces
selenious acid, H2SeO3.
Selenium forms a variety of compounds in which the selenium atom
is bonded to both an oxygen and a halogen atom. A notable example
is selenium(VI) oxychloride, SeO2Cl2, an extremely powerful solvent.
The most important acid of selenium is selenic acid, H2SeO4, which
is as strong as sulfuric acid and more easily reduced. atomic number
34 atomic weight 78.96 masses of stable isotopes 74, 76, 77, 78, 80,
82 melting point amorphous 50 C (122 F) gray 217 C (423 F)
boiling point 685 C (1,265 F) density amorphous 4.28 g/cm3 gray
4.79 g/cm3 oxidation states -2, +4, +6 electron config. 2-8-18-6 or
1s22s22p63s23p6 3d104s24p4
"selenium" Encyclop�dia Britannica Online.
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