Part 1: Its chemistry and occurrence Mickey Sarquis Miami University Middletown, Ohio 45042 Charles D. Mickey Texas A8M University at Gaiveston Galveston, Texas 77553
edited by: Michael R. Slabaugh, Weber State College. Ogden. UT 84408
Seleniun~was discowred i n ld17, by Herzelillh and Cahn in the sedimvnt taken from the lead rhamlwr uf a sulfuric arid plant in Gripsholm, Sweden. Its name is derived from the Greek word selene, for moon, because of its chemical similarity to tellurium which, in 1782, had been named for the earth (Latin tellus, earth). The Element Selenium exists in several allotropic forms: amorphous, crystalline, and metallic. The amorphous form is a dark-red to black powder. I t softens a t 50-6O0C, and becomes elastic a t 7O0C. The dark-red transparent crystalline form has the only sharp melting point of these allotropic forms, m.p. 144°C. These red crystals are metastable and will change upon heating, to the more stable gray form. In the gray metallic form, selenium appears as gray to black lustrous hexagonal crvstals which melt a t 217°C. The erav. form conducts electricity and rectifies alternating current. Upon exposure to light. its conductivitv can be increased uo to one thousand tikes. The electronicand photoelectric of metallic selenium are the basis for many industrial uses of this element. Some of the properties of elemental Se include: Atomic weight Atomic number Covalent radius A Atomic radius, A Ionic radius, A Electronegativity Electronic structure
1 S R Y
. .
The naturally occurring isotopes of selenium include soSe (49.82%), (23.52701, 76Se (9.02961, W e (9.19%), %e (7.58%), and W e (0.87%). None of the naturally occurring isotopes of selenium are radioactive. The radioisotopes are by-products of the nuclear reactor and neutron activation technology. Selenium-75 is one of the more useful radionuclides. It is widelv used in hioloeical tracer exoeriments and diagnostic p r o ( ~ i 1 1 1 In~ilust~ r ~ ~ . i t has s ronvenimt gnmm:t-ray I'or n,untinrr" and its hnli-liie 11 L(ldnvs) is more that) adequare to allow complete chemical separation from other activities (I). Selenium, like sulfur, exists in several oxidation states, viz., -2.0, +2 (not yet found in nature), +4, and +6. Consequently these elements have some analogous properties though there are some prominent differences. The chemical diversity of selenium's multiple oxidation states is a major factor affecting its behavior in the environment. The following review of these properties is a brief summary of the more comprehensive presentations ( 1 , Z ) . This feature presents relevant applications of chemistry to every day life. The information presented might be used directly in class, posted on hulletin hoards or otherwise used to stimulate student involvement in activities related to chemistry. Contributions should be sent to the feature editor.
886 1 Journal of Chemical Education
Properties Selenide (SeZ-) In the -2 oxidation state, selenium occurs as hydrogen selenide, metal selenides and a number of organic selenides. Aqueous hydrogen selenide is a moderately strong acid (Klz5 = 1.3 X lo-'; Kzz5 = 1 X 10-'l). Gaseous hydrogen selenide is fifteen times more dangerous than hydrogen sulfide, based on threshold limit values, however, hydrogen selenide has never caused a death or an illness lasting more than ten days in a human. There are three reasons: first, the use of hydrogen selenide is limited to small volumes by industrial and research workers. The second reason is that hydrogen selenide is very easily oxidi7ed to nowtrnir elrrnmtal red selmiurn u n the mucous memhanesor thv nwrilna in rht!alwdi ufthe lungs. 'l'hinlly, the udor
