are produced from solutions of black selenium by varying the evaporation rate of the solvent (usually CS2). They all have a relatively low, monoclinic crystal symmetry (space group 14) and contain nearly identical puckered cyclooctaselenium (Se8) rings with different geometric arrangements, as in sulfur. The eight atoms of a ring are not equivalent (i.e. 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. Five of these, 74Se, 76Se, 77Se, 78Se, 80Se, are stable, with 80Se being the most abundant (49.6% natural abundance). Also naturally occurring is the long-lived primordial radionuclide 82Se, with a half-life of 9.2×1019 years. The non-primordial radioisotope 79Se also occurs in minute quantities in uranium ores as a product of nuclear fission. Selenium also has numerous unstable synthetic isotopes ranging from 64Se to 95Se; the most stable are 75Se with a half-life of 119.78 days and 72Se with a half-life of 8.4 days. Isotopes lighter than the stable isotopes