The elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At), which make up the seventh period in the periodic table of the elements. The first four members of the period are present in trace levels in the atmosphere as the result of both natural and anthropogenic activity. The chemistry of chlorine compounds and bromine compounds is particularly important in the catalytic destruction of ozone in the stratosphere and is responsible for the occurrence of the antarctic ozone hole each spring. Because of this role in ozone depletion, the production of chlorinated and brominated organic compounds is being phased out.