Platinum is one of six platinum-group elements that include palladium, iridium, osmium, rhodium, and ruthenium. While chemically similar, their physical properties vary, with platinum, iridium, and osmium being the densest known metals. Platinum and palladium are highly resistant to heat and corrosion and are soft and ductile, while rhodium and iridium are more difficult to work with ruthenium and osmium being hard and brittle. All platinum-group elements can act as catalysts in industrial applications when alloyed with each other or other metals.