section_path: Compounds/Oxides and chalcogenides
section_name: Oxides and chalcogenides
paragraph_text: Oxides and chalcogenides
  
  Silver and gold have rather low chemical affinities for oxygen, lower than copper, and it is therefore expected that silver oxides are thermally quite unstable. Soluble silver(I) salts precipitate dark-brown silver(I) oxide, Ag O, upon the addition of alkali. (The hydroxide AgOH exists only in solution; otherwise it spontaneously decomposes to the oxide.) Silver(I) oxide is very easily reduced to metallic silver, and decomposes to silver and oxygen above 160 °C. This and other silver(I) compounds may be oxidized by the strong oxidizing agent peroxodisulfate to black AgO, a mixed silver(I,III) oxide of formula Ag Ag O. Some other mixed oxides with silver in non-integral oxidation states, namely Ag O and Ag O, are also known, as is Ag O which behaves as a metallic conductor.
  
  Silver(I) sulfide, Ag S, is very readily formed from its constituent elements and is the cause of the black tarnish on some old silver objects. It may also be formed from the reaction of hydrogen sulfide with silver metal or aqueous Ag ions. Many non-stoichiometric selenides and tellurides are known; in particular, AgTe is a low-temperature superconductor.