group elements except the noble gases. Usually, their stoichiometries follow the classical valence trends, e.g. SiS2, B2S3, Sb2S3. Many exceptions exist however, e.g. P4S3 and S4N4. The structures of many main group materials are dictated by directional covalent bonding, rather than by close packing. The chalcogen is assigned positive oxidation states for the halides, nitrides, and oxides. See also Carbon dichalcogenide Chalcogen Chalcogenide glass Hydrogen chalcogenide Negative resistance Phase-change memory References External links Advanced Chalcogenide Technologies and Applications Lab ACTAlab Jun 14, 2016 Phase change memory-based 'moneta' system points to the future of computer storage ScienceBlog Jun 03, 2011 Big Blue boffins hatch dirt-cheap solar cells The Register, 12 February 2010