-Rh- is a chemical element with atomic number 45. Rhodium is a rare, silvery-white, hard and chemically inert transition metal and a member of the platinum group. Rhodium is composed of only one isotope, 103Rh. Naturally occurring rhodium is found as a free metal, alloyed with similar metals, and never as a chemical compound. Rhodium is one of the rarest precious metals, and the most costly.