<unk>  is the anion<unk> −.  It is important as the product of the one-electron reduction of dioxygen, which occurs widely in nature. With one unpaired electron, the<unk>  ion is a free radical, and, like dioxygen, it is paramagnetic.
Superoxide
Peroxide
Hydrogen peroxide 
Hydrogen peroxide
Hydrogen sulfide
Sulfur dioxide
Carbon dioxide
Nitrogen oxide