text: can sulfide exist in nature in an uncombined state? what is the chemical structure of sulfide and how can in exist as s^2- ? any help is much appreciated. the term sulfide (sulphide in british english) refers to several types of chemical compounds containing sulfur in its lowest oxidation number of −2.  formally, "sulfide" is the dianion, s2−, which exists in strongly alkaline aqueous solutions formed from h2s or alkali metal salts such as li2s, na2s, and k2s. sulfide is exceptionally basic and, with a pka > 14, it does not exist in appreciable concentrations even in highly alkaline water. instead, sulfide combines with protons to form hs−, which is variously called hydrogen sulfide ion, hydrosulfide ion, sulfhydryl ion, or bisulfide ion. at still lower ph's (<7), hs− converts to h2s, hydrogen sulfide. thus, the exact sulfur species obtained upon dissolving sulfide salts depends on the ph of the final solution.