A group of elements in the periodic table: oxygen (O), sulphur (S), selenium (Se), tellurium (Te), and polonium (Po), which all have outer electronic configurations ns2np4 with no partly filled inner levels. They are also called the chalcogens. Formerly, they were classified in group VI, which consisted of two subgroups: group VIB (the main group) and group VIA. Group VIA consisted of chromium (Cr), molybdenum (Mo), and tungsten (W), which now form group 6 are generally classified with the transition elements.
The configurations are just two electrons short of the configuration of a noble gas and the elements are characteristically electronegative and almost entirely nonmetallic. Ionization energies are high, (O 1314 to Po 813 kJ mol−1) and monatomic cations are not known. Polyatomic cations do exist, e.g. O2+, S82+, Se82+, Te42+. Electronegativity decreases down the group but the nearest approach to metallic character is the occurrence of ‘metallic’ allotropes of selenium, tellurium, and polonium along with some metalloid properties, in particular, marked photoconductivity. The elements of group 16 combine with a wide range of other elements and the bonding is largely covalent. The elements all form hydrides of the type XH2. Apart from water, these materials are all toxic foul-smelling gases; they show decreasing thermal stability with increasing relative atomic mass of X. The hydrides dissolve in water to give very weak acids (acidity increases down the group). Oxygen forms the additional hydride H2O2 (hydrogen peroxide), but sulphur forms a range of sulphanes, such as H2S2, H2S4, H2S6.
Oxygen forms the fluorides O2F2 and OF2, both powerful fluorinating agents; sulphur forms analogous fluorides along with some higher fluorides, S2F2, SF2, SF4, SF6, S2F10. Selenium and tellurium form only the higher fluorides MF4 and MF6; this is in contrast to the formation of lower valence states by heavier elements observed in groups 13, 14, and 15. The chlorides are limited to M2Cl2 and MCl4; the bromides are similar except that sulphur only forms S2Br2. All metallic elements form oxides and sulphides and many form selenides.