ABSTRACT
Geochemical and biochemical characteristics of the elements in Group VII are widely diverging. Trace elements of the Subgroup VIIa (new group 17) the halogens (F, Cl, Br, and I), are much less common in the biosphere than Cl which some authors include also as a trace element. The last element of this subgroup, astatine (At), occurs as radioactive isotopes of very short lives. In nature, halogens form simple anions or, except F, combine with oxygen anions and play a significant role in biochemical processes. These anions are highly mobile in the Earth’s crust although they are considered to reveal lithophile properties.
