The eight atoms of a ring are not equivalent (i.e. They all have a relatively low, monoclinic crystal symmetry ( space group 14) and contain nearly identical puckered cyclooctaselenium (Se 8) rings with different geometric arrangements, as in sulfur.
The red α, β, and γ forms are produced from solutions of black selenium by varying the evaporation rate of the solvent (usually CS 2). Upon heating, it softens at 50 ☌ and converts to gray selenium at 180 ☌ the transformation temperature is reduced by presence of halogens and amines. Black selenium is a brittle, lustrous solid that is slightly soluble in CS 2. The structure of black selenium is irregular and complex and consists of polymeric rings with up to 1000 atoms per ring. 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. Both units are derived from the carbon-12 isotope, as 12 u is the exact atomic mass of that isotope.Selenium is a chemical element it has the symbol Se and atomic number 34. The atomic mass is usually measured in the units unified atomic mass unit (u), or dalton (Da). These concepts are further explained below.
Mass of molecule calculated from the mass of its isotopes (in contrast of measured ba a mass spectrometer) Integer mass of molecule consisting of most abundant isotopes Ratio of mass m of a molecule and and the atomic mass constant m u Ratio of mass m and and the atomic mass constant m u Mass Concepts in Chemistry name in chemistry Note that the former is now often referred to as the "molecular weight" or "atomic weight". The former usually implies a certain isotopic distribution, whereas the latter usually refers to the most common isotope ( 16O 2). For example, the macroscopic mass of oxygen (O 2) does not correspond to the microscopic mass of O 2. This means that from a physical stand point, these mixtures are not pure. On the macroscopic level, most mass measurements of pure substances refer to a mixture of isotopes. In addition, the situation is rendered more complicated by the isotopic distribution. The name "atomic mass" is used for historical reasons, and originates from the fact that chemistry was the first science to investigate the same physical objects on macroscopic and microscopic levels. "Neither the name of the physical quantity, nor the symbol used to denote it, should imply a particular choice of unit."