third transition series

The metals of the third transition series, a group of elements characterized by the filling of an inner d electron orbital as atomic number increases, begin with hafnium and include tantalum, tungsten, rhenium, osmium, iridium, platinum, and gold. Many of the chemical and physical properties of the transition elements are due to their unfilled d orbitals. The third-series transition metals, like the elements in the second transition series for a given group, have chemical properties similar to those of the element in the first series. As is the case with the second-series metals, the third-series metals exhibit definite differences from the lighter element of the group, behaving much more like those of the second series (q. v.). In general, higher oxidation states for the heavier transition elements are generally more stable than is the case for the elements in the first transition series; this is true not only, as has been mentioned in the text for the second series, for the properties of the oxo anions but equally true for the higher halides. Thus the heavier elements form such compounds as tungsten(VI) chloride, WCl6; platinum(VI) fluoride, PtF6, etc., which have no analogs among the first-series elements. The chemistry of aquo ions of lower valence states, especially +2 and +3, is relatively unimportant for most of the heavier transition elements, in contrast to its predominance in the chemistry of the lighter metals.