Iridium Iridium () is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can even be flammable.