Selenium
Selenium is quite rare (9 10 ^6 % of the earth’s crust). It occurs mainly as an impurity
in sulfur, sulfide, and sulfate deposits. It is obtained from the flue dusts that result from
roasting sulfide ores and from the “anode mud” formed in the electrolytic refining of
copper. It is used as a red coloring in glass. The gray crystalline allotropic form of sele-
nium has an electrical conductivity that is very light-sensitive, so it is used in photocopy
machines and in solar cells.
Tellurium
Tellurium is even less abundant (2 10 ^7 % of the earth’s crust) than selenium. It occurs
mainly in sulfide ores, especially with copper sulfide, and as the tellurides of gold and
silver. It, too, is obtained from the “anode mud” from refining of copper. The element
forms brass-colored, shiny, hexagonal crystals having low electrical conductivity. It is added
to some metals, particularly lead, to increase electrical resistance and improve resistance
to heat, corrosion, mechanical shock, and wear.
REACTIONS OF GROUP VIA ELEMENTS
Some reactions of the Group VIA elements are summarized in the following table.
General Equation RemarksxEyM88nMyEx With many metals
zEMxEy88nMxEyz Especially with S, Se
EH 2 88nH 2 E Decreasingly in the series O 2 , S, Se, Te
E3F 2 88nEF 6 With S, Se, Te, and excess F 2
2ECl 2 88nE 2 Cl 2 With S, Se (Te gives TeCl 2 ); also with Br 2
E 2 Cl 2 Cl 2 88n2ECl 2 With S, Se; also with Br 2
E2Cl 2 88nECl 4 With S, Se, Te, and excess Cl 2 ; also with Br 2
EO 2 88nEO 2 With S (with Se, use O 2 NO 2 )
24-9
(Above)Yellow crystalline sulfur contains S 8 rings.
(Right)When molten sulfur is heated above 150°C, an amorphous
form of sulfur called “plastic sulfur” is formed as the S 8 rings
break and the fragments link to form long chains.
E represents a Group VIA element.