356Isotopes
Selenium has six naturally occurring isotopes, five of which are stable:^74 Se,^76 Se,^77 Se,
(^78) Se, and (^80) Se. The last three also occur as fission products, along with (^79) Se, which has a
half-life of 327,000 years. The final naturally occurring isotope,^82 Se, has a very long half-
life (~10^20 yr, decaying via double beta decay to^82 Kr), which, for practical purposes, can
be considered to be stable. Twenty-three other unstable isotopes have been
characterized.
See also Selenium-79 for more information on recent changes in the measured half-life of
this long-lived fission product, important for the dose calculations performed in the frame
of the geological disposal of long-lived radioactive waste.
Chemical Compounds
Selenium compounds commonly exist in the oxidation states −2, +2, +4, and +6.
Chalcogen Compounds
Selenium forms two oxides: selenium dioxide (SeO 2 ) and selenium trioxide (SeO 3 ).
Selenium dioxide is formed by the reaction of elemental selenium with oxygen:
Se 8 + 8 O 2 → 8 SeO 2
It is a polymeric solid that forms monomeric SeO 2 molecules in the gas phase. It dissolves
in water to form selenous acid, H 2 SeO 3. Selenous acid can also be made directly by
oxidizing elemental selenium with nitric acid:
3 Se + 4 HNO 3 + H 2 O → 3 H 2 SeO 3 + 4 NO
Unlike sulfur, which forms a stable trioxide, selenium trioxide is thermodynamically
unstable and decomposes to the dioxide above 185 °C:
2 SeO 3 → 2 SeO 2 + O 2 (ΔH = −54 kJ/mol)
Selenium trioxide is produced in the laboratory by the reaction of anhydrous potassium
selenate (K 2 SeO 4 ) and sulfur trioxide (SO 3 ).
Salts of selenous acid are called selenites. These include silver selenite (Ag 2 SeO 3 ) and
sodium selenite (Na 2 SeO 3 ).
Hydrogen sulfide reacts with aqueous selenous acid to produce selenium disulfide:
H 2 SeO 3 + 2 H 2 S → SeS 2 + 3 H 2 O
Selenium disulfide consists of 8-membered rings of a nearly statistical distribution of sulfur
and selenium atoms. It has an approximate composition of SeS 2 , with individual rings
varying in composition, such as Se 4 S 4 and Se 2 S 6. Selenium disulfide has been use in
shampoo as an anti-dandruff agent, an inhibitor in polymer chemistry, a glass dye, and a
reducing agent in fireworks.