they are not mapped one onto another by any symmetry operation), and in fact in the γ-monoclinic form, half the rings are in one configuration (and its mirror image) and half in another. The packing is most dense in the α form. In the Se8 rings, the Se-Se distance varies depending on where the pair of atoms is in the ring, but the average is 233.5 pm, and the Se-Se-Se angle is on average 105.7° . Other selenium allotropes may contain Se6 or Se7 rings.The most stable and dense form of selenium is gray and has a chiral hexagonal crystal lattice (space group 152 or 154 depending on the chirality) consisting of helical polymeric chains, where the Se-Se distance is 237.3 pm and Se-Se-Se angle is 103.1° . The minimum distance between chains is 343.6 pm. Gray Se is formed by mild heating of other allotropes, by slow cooling of molten Se, or by condensing Se vapor just below the melting point. Whereas other Se forms are insulators, gray Se is a semiconductor showing appreciable photoconductivity. Unlike the other allotropes, it is insoluble in CS2. It resists oxidation by air and is not attacked by nonoxidizing acids. With strong reducing agents, it forms polyselenides. Selenium does not exhibit the changes in viscosity that sulfur undergoes when gradually heated. Optical properties
Owing to its use as a photoconductor in flat-panel x-ray detectors (see below), the optical properties of amorphous selenium (α-Se) thin films have been the subject of intense research. Isotopes
Selenium has seven naturally occurring isotopes.
Alloys
Selenium is used with bismuth in brasses to replace more toxic lead. The regulation of lead in drinking water applications such as in the US with the Safe Drinking Water Act of 1974, made a reduction of lead in brass necessary. The new brass is marketed under the name EnviroBrass. Like lead and sulfur, selenium improves the machinability of steel at concentrations around 0.15%. Selenium produces the same machinability improvement in copper alloys. Lithium–selenium batteries
The lithium–selenium (Li–Se) battery is one of the most promising systems for energy storage in the family of lithium batteries. The Li–Se battery is an alternative to the lithium–sulfur battery, with an advantage of high electrical conductivity. Solar cells
Selenium was the basis of the very first solar cells, with the first example of rooftop solar being a selenium cell from 1884. Such cells were later used in battery-free light meters for photography. Copper indium gallium selenide is a material used in solar cells. Photoconductors
Amorphous selenium (α-Se) thin films have found application as photoconductors in flat panel x-ray detectors. These detectors use amorphous selenium to capture and convert incident x-ray photons directly into electric charge. Rectifiers
Selenium rectifiers were first used in 1933. Their use continued into the 1990s. Other uses
Small amounts of organoselenium compounds have been used to modify the catalysts used for the vulcanization for the production of rubber.The demand for selenium by the electronics industry is declining. Its photovoltaic and photoconductive properties are still useful in photocopying, photocells, light meters and solar cells.