Selenium is the first member of the group 16 elements O, S, Se, Te, and Po that shows some metallic character. The increasing metallic character in the series O, S, Se, Te, Po is reflected also in the lower electronegativity, in the less acid character of the oxides, and in the lower stability of the oxidation number +VI for selenium compared to sulfur. There is a large variety of compounds of SeII, SeIV, and SeVI with the electronegative elements N, O, F, Cl, and Br. Thus, there are six general types of compounds, and these will comprise the main sections in the article. They are cations, halides, oxides, haloselenates, oxohaloselenates, and the related compounds that form when both selenium and nitrogen are present in the molecule.