The six platinum group metals (pgms: ruthenium, rhodium, palladium, osmium, iridium and platinum) posed a number of problems for 19th-century chemists, including Mendeleev, for their Periodic classification. This account discusses the discovery of the pgms, the determination of their atomic weights and their classification.

The Periodic Table has the column of the noble gas atoms (He, Ne, Ar, Kr, Xe, Rn) as one of its main pillars. Indeed the inert chemical nature of their closed shell structure is so striking that it is sometimes extended to all such structures. Is it true however that any closed shell, say a closed d-subshell will denote a lack of chemical activity? Take the noble metals for instance, so renowned for their catalytic capacity. Platinum has 10 electrons in (...) its valence shell which makes one of its excited states to be a closed 5d10–6s0 state. Surely this state would not be expected to be crucial to the catalytic activity of platinum, or would it? Or take palladium whose ground state is precisely the 4d10–5s0 state, should we expect that an isolated Pd atom at near zero-point temperature would attack a closed-shell hydrogen molecule efficiently? We shall here show that this is precisely the case; the closed-shell excited states of nickel and platinum are indeed crucial, through symmetry avoided crossings, for their reactivity. Other valuable catalysts as ruthenium depend on their excited states with maximal d-shell occupancy for their activity. The most notable confirmation of this new finding; that closed d-shells are vital to the catalytic activity of noble metals however, is the case of palladium whose closed-shell ground state is indeed capable of attacking hydrogen and hydrocarbon molecules even at temperatures well below 10 K as was predicted theoretically and immediately confirmed experimentally. (shrink)