primarily undergo beta plus decay to isotopes of arsenic, and isotopes heavier than the stable isotopes undergo beta minus decay to isotopes of bromine, with some minor neutron emission branches in the heaviest known isotopes. Chemical compounds Selenium compounds commonly exist in the oxidation states −2, +2, +4, and +6. Chalcogen compounds Selenium forms two oxides: selenium dioxide (SeO2) and selenium trioxide (SeO3). Selenium dioxide is formed by the reaction of elemental selenium with oxygen: Se8 + 8 O2 -> 8 SeO2 It is a polymeric solid that forms monomeric SeO2 molecules in the gas phase. It dissolves in water to form selenous acid, H2SeO3. Selenous acid can also be made directly by oxidizing elemental selenium with nitric acid: 3 Se + 4 HNO3 + H2O -> 3 H2SeO3 + 4 NO Unlike sulfur, which forms a stable trioxide, selenium trioxide is thermodynamically unstable and decomposes to the dioxide above 185 °C: 2 SeO3 -> 2 SeO2 + O2 (ΔH = −54 kJ/mol) Selenium trioxide is produced in the laboratory by the reaction of anhydrous potassium selenate (K2SeO4) and sulfur trioxide (SO3). Salts of selenous acid are called selenites. These include silver selenite (Ag2SeO3) and sodium selenite (Na2SeO3). Hydrogen sulfide reacts with aqueous selenous acid to produce selenium disulfide: H2SeO3 + 2 H2S -> SeS2 + 3 H2O Selenium disulfide consists of 8-membered rings. It has an approximate composition of SeS2, with individual rings varying in composition, such as Se4S4 and Se2S6. Selenium disulfide has been used in shampoo as an antidandruff agent, an inhibitor in polymer chemistry, a glass dye, and a reducing agent in fireworks. Selenium trioxide may be synthesized by dehydrating selenic acid, H2SeO4, which is itself produced by the oxidation of selenium dioxide with hydrogen peroxide: SeO2 + H2O2 -> H2SeO4 Hot, concentrated selenic acid can react with gold to form gold(III) selenate. Halogen compounds Iodides of selenium are not well known. The only stable chloride is selenium monochloride (Se2Cl2), which might be better known as selenium(I) chloride; the corresponding bromide is also known. These species are structurally analogous to the corresponding disulfur dichloride. Selenium dichloride is an important reagent in the preparation of selenium compounds (e.g. the preparation of Se7). It is prepared by treating selenium with sulfuryl chloride (SO2Cl2). Selenium reacts with fluorine to form selenium hexafluoride: Se8 + 24 F2 -> 8 SeF6 In comparison with its sulfur counterpart (sulfur hexafluoride), selenium hexafluoride