Osmium powder has the characteristic smell of osmium tetroxide. Unlike ruthenium and osmium, rhodium forms no volatile oxygen compounds. The platinum group consists of platinum, iridium, osmium, palladium, rhodium, and ruthenium. The six platinum-group metals are ruthenium, rhodium, palladium, osmium, iridium, and platinum. Ruthenium, osmium, and iridium are considered the minor platinum group metals. Examples of such solutes are noble gases and osmium tetroxide. The properties of ruthenium and osmium compounds are often similar. Rhenium prefers to enter sulfides more readily than osmium. Osmium tetroxide forms red osmates upon reaction with a base. Like palladium, powdered osmium effectively absorbs hydrogen atoms. Some stains, such as osmium tetroxide and osmium ferricyanide, are very chemically active. This same trend appears for ruthenium but not osmium. It is also one of the densest, exceeded only by platinum, iridium and osmium. His version included a "group b" consisting of oxygen, sulfur, selenium, tellurium, and osmium. Uranium, however, reacts to its nitride during catalysis and osmium oxide is rare.