Iridium is a
chemical element with symbol
Ir and
atomic number 77. A very hard, brittle, silvery-white
transition metal of the
platinum group, iridium is generally credited with being the second densest element (after
osmium) based on measured density, although calculations involving the space lattices of the elements show that iridium is denser. It is also the most
corrosion-resistant metal, even at temperatures as high as 2000 °C. Although only certain molten salts and
halogens are corrosive to solid iridium, finely divided iridium dust is much more reactive and can be flammable.