Occurrence

Group 16. The elements oxygen ($_{8}$O), sulfur ($_{16}$S), selenium ($_{34}$Se), tellurium ($_{52}$Te) and polonium ($_{84}$Po) constitute Group 16, called the oxygen family. A large number of metal ores are oxides or sulfides, so group 16 elements are also called chalcogens, or ore-forming elements.

Oxygen is the most abundant of all the elements on earth: it forms 20.95 % by volume of air and about 46.6 % by mass of the earth's crust. Sulfur forms 0.034 % by mass of the earth's crust. It occurs mainly in combined forms — as sulfates such as gypsum ($\mathrm{CaSO_4{\cdot}2H_2O}$), epsom salt ($\mathrm{MgSO_4{\cdot}7H_2O}$) and baryte ($\mathrm{BaSO_4}$), and as sulfides such as galena (PbS), zinc blende (ZnS) and copper pyrites ($\mathrm{CuFeS_2}$). Selenium and tellurium are also found as metal selenides and tellurides in sulfide ores. Polonium, which is radioactive, is a decay product of thorium and uranium.

Group 17. Fluorine ($_{9}$F), chlorine ($_{17}$Cl), bromine ($_{35}$Br), iodine ($_{53}$I) and astatine ($_{85}$At) constitute Group 17. These are collectively known as halogens (Greek halo means salt, gene means born) — that is, salt-producing elements. Halogens are very reactive owing to their high electronegativities, and hence they are not found in the free state; they occur in the form of compounds. Fluorine occurs mainly as insoluble fluorides — fluorspar ($\mathrm{CaF_2}$), cryolite ($\mathrm{Na_3AlF_6}$) and fluorapatite ($\mathrm{3Ca_3(PO_4)_2{\cdot}CaF_2}$) — and small quantities are present in soil, fresh water, plants, and the bones and teeth of animals. Sea water contains chlorides, bromides and iodides of Na, K, Mg and Ca, but mainly NaCl (2.5 % by mass). The deposits of dried-up sea beds contain sodium chloride and carnallite, $\mathrm{KCl{\cdot}MgCl_2{\cdot}6H_2O}$. Marine life also carries iodine in its systems — sea weed, for example, contains up to 0.5 % iodine — and chile saltpetre contains up to 0.2 % of sodium iodate. Astatine, the last member of the halogen family, is radioactive, with a half life of 8.1 hours. …