The platinum metals consist of a group of six closely related rare metals: rhodium (Rh), ruthenium (Ru), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt), which is the most important member of the group. The first three occur at the end of the Second series of Transition Elements in the Periodic Table, before silver. The second three, which occur at the end of the third transition series, precede gold and are among the densest known substances, having densities between 21.4 and 22.4 times that of water.
Platinum and palladium are the most abundant of the six metals. Platinum was the earliest known, discovered by the Spaniards in the South American silver mines about 1550. The Spanish called it platina – little silver. They recognized it as being different from true silver because it could not be melted – platinum has a melting point of 3225° F (1774° C).
It also caused difficulties for the Spaniards, as its density was close to that of gold,ind when gold-plated it was virtually indistin;uishable from the solid metal. The properties of platinum were first seriously studied in the midAghteenth century by Watson, Scheffer and Morggarf. In 1803 W. H. Wollaston showed that all platinum previously examined was in fact an alloy mith similar metals.
He managed to isolate two; one he named palladium for Pallas, a newly discovered ninor planet, and the other rhodium from the Ilreek word rhodon, rose, because some of its compounds are rose-colored. One year later Tennant liscovered two more of the platinum metals, namng them osmium from the Greek osme, meaning a ;men, because its oxide has an unpleasant odor, and ridium from iris, the Greek word for rainbow. In 1845 Claus completed the sextet with the discovery of ruthenium, which he named for a province of his lative Russia.