When rapidly melted, it forms the black, vitreous form, usually sold commercially as beads. When prepared in chemical reactions, selenium is usually an amorphous, brick-red powder. Selenium forms several allotropes that interconvert with temperature changes, depending somewhat on the rate of temperature change. Characteristics Physical properties Structure of hexagonal (gray) selenium Selenium requirements in plants differ by species, with some plants requiring relatively large amounts and others apparently not requiring any. Selenium is listed as an ingredient in many multivitamins and other dietary supplements, as well as in infant formula, and is a component of the antioxidant enzymes glutathione peroxidase and thioredoxin reductase (which indirectly reduce certain oxidized molecules in animals and some plants) as well as in three deiodinase enzymes. Selenium is still used in a few types of DC power surge protectors and one type of fluorescent quantum dot.Īlthough trace amounts of selenium are necessary for cellular function in many animals, including humans, both elemental selenium and (especially) selenium salts are toxic in even small doses, causing selenosis. ![]() Applications in electronics, once important, have been mostly replaced with silicon semiconductor devices. Selenium is a semiconductor and is used in photocells. The chief commercial uses for selenium today are glassmaking and pigments. Minerals that are pure selenide or selenate compounds are known but rare. Commercially, selenium is produced as a byproduct in the refining of these ores, most often during production. Selenium is found in metal sulfide ores, where it partially replaces the sulfur. Selenium (from Ancient Greek σελήνη ( selḗnē) 'moon') was discovered in 1817 by Jöns Jacob Berzelius, who noted the similarity of the new element to the previously discovered tellurium (named for the Earth). It seldom occurs in its elemental state or as pure ore compounds in Earth's crust. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and tellurium, and also has similarities to arsenic. One Latin connotation for the word “mole” is “large mass” or “bulk,” which is consistent with its use as the name for this unit.Selenium is a chemical element it has the symbol Se and atomic number 34. It provides a specific measure of the number of atoms or molecules in a sample of matter. The mole is an amount unit similar to familiar units like pair, dozen, gross, etc. This experimental approach required the introduction of a new unit for amount of substances, the mole, which remains indispensable in modern chemical science. Today, sophisticated instruments allow the direct measurement of these defining microscopic traits however, the same traits were originally derived from the measurement of macroscopic properties (the masses and volumes of bulk quantities of matter) using relatively simple tools (balances and volumetric glassware). ![]() ![]() However, because a hydrogen peroxide molecule contains two oxygen atoms, as opposed to the water molecule, which has only one, the two substances exhibit very different properties. For example, water, H 2O, and hydrogen peroxide, H 2O 2, are alike in that their respective molecules are composed of hydrogen and oxygen atoms. The identity of a substance is defined not only by the types of atoms or ions it contains, but by the quantity of each type of atom or ion. The few exceptions to this guideline are very light ions derived from elements with precisely known atomic masses. Even when calculating the mass of an isolated ion, the missing or additional electrons can generally be ignored, since their contribution to the overall mass is negligible, reflected only in the nonsignificant digits that will be lost when the computed mass is properly rounded. Moreover, the mass of an electron is negligibly small with respect to the mass of a typical atom. Even though a sodium cation has a slightly smaller mass than a sodium atom (since it is missing an electron), this difference will be offset by the fact that a chloride anion is slightly more massive than a chloride atom (due to the extra electron). This approach is perfectly acceptable when computing the formula mass of an ionic compound. Note that the average masses of neutral sodium and chlorine atoms were used in this computation, rather than the masses for sodium cations and chlorine anions. \): Table salt, NaCl, contains an array of sodium and chloride ions combined in a 1:1 ratio.
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