ABSTRACT
The 17th group of the Periodic Table is composed of three trace elements: uorine (F), bromine (Br), and iodine (I). To this group of elements belongs chlorine (Cl), which is relatively common in the biosphere, but some authors have treated it as a trace element. I is the least reactive of the elements in this group. The last element of this group, astatine (At), occurs as radioactive unstable isotopes of very short lives (the longest half-life of 209At is 8.1 h).
