2. Astate-211

2.1 Features

Astatine (Z = 85) is the heaviest of the halogens. It exists only in the form of unstable isotopes with short half-lives (from 125 ns to 8.1 h for the 32 known isotopes). In its natural state, only traces, estimated at a few grams in total, are present throughout the earth's crust, resulting from the decay of heavy elements such as uranium. Although some of its isotopes can be produced artificially, the quantities of astatine obtained to characterize it remain too small (in the picogram range at best) to make it possible to assess its physical or chemical characteristics by conventional means, notably with the spectroscopic tools currently available. Most of this element's properties have therefore been estimated indirectly, often by extrapolation from its better-known...