Introduction group VIIB

Group VIIB includes following elements: Manganese - [Mn], Technetium - [Tc], Rhenium - [Re].

In terms of history, abundance and availability, it is difficult to imagine a greater contrast than exists in this group between manganese and its congeners, technetium and rhenium. Millions of tonnes of manganese are used annually, and its most common mineral, pyrolusite, has been used in glassmaking since the time of the Pharaohs. On the other hand, technetium and rhenium are exceedingly rare and were only discovered comparatively recently, the former being the first new element to have been produced artificially and the latter being the last naturally occurring element to be discovered.

Metallic manganese was first isolated in 1774 when C. W. Scheele recognized that pyrolusite contained a new element, and his fellow Swede, J. G. Gahn, heated the MnO2 with a mixture of charcoal and oil. The purity of this sample of the metal was low, and high-purity (99.9%) manganese was only produced in the 1930s when electrolysis of MnII solutions was used. In Mendeleev's table, this group was completed by the then undiscovered eka-manganese (Z = 43) and dvi-manganese (Z = 75). Confirmation of the existence of these missing elements was not obtained until H. G. J. Moseley had introduced the method of X-ray spectroscopic analysis. Then in 1925 W. Noddack, I. Tacke (later Frau Noddack) and O. Berg discovered element 75 in a sample of gadolinite (a basic silicate of beryllium, iron and lanthanides) and named it rhenium after the river Rhine. The element was also discovered, independently by F. H. Loring and J. F. G. Druce, in manganese compounds, but is now most usually recovered from the flue dusts produced in the roasting of Cu-Mo ores.

The Noddacks also claimed to have detected element 43 and named it masurium after Masuren in Prussia. This claim proved to be incorrect, however, and the element was actually detected in 1937 in Italy by C. Perrier and E. Segr6 in a sample of molybdenum which had been bombarded with deuterons in the cyclotron of E. O. Lawrence in California. It was present in the form of the β- emitters 95Tc and 97Tc with half-lives of 61 and 90 days respectively. The name technetium (from Greek - artificial) is clearly appropriate even though minute traces of the more stable 99Tc (half-life = 2.11·105 y) do occur naturally as a result of spontaneous fission of uranium.