The group 16 elements

   The group 16 elements – oxygen, sulfur, selenium, tellurium      and polonium – are called the chalcogens  Oxygen occupies so central a position in any treatment of inorganic chemistry that discussions of many of its compounds are dealt with under other elements. The decrease in non-metallic character down the group is easily recognized in the elements:

• oxygen exists only as two gaseous allotropes (O₂ and O₃);
• sulfur has many allotropes, all of which are insulators;
• the stable forms of selenium and tellurium are semiconductors;
• polonium is a metallic conductor.

    Knowledge of the chemistry of Po and its compounds is limited because of the absence of a stable isotope and the difficulty of working with ²¹⁰Po, the most readily available isotope.   

  Polonium-210 is produced from ²⁰⁹Bi by an (n,γ) reaction  followed by b-decay of the product. It is an intense a-emitter (t_1/2 = 138 days) liberating 520 kJ g^-1 h^-1, and is a lightweight source of energy in space satellites.   However, this large energy loss causes many compounds of Po to decompose; Po decomposes water, making studies of chemical reactions in aqueous solution difficult. Polonium is a metallic conductor and crystallizes in a simple cubic lattice.  It forms volatile, readily hydrolysed halides PoCl₂, PoCl₄ , PoBr₂, PoBr₄ and PoI₄ and complex ions [PoX₆]^2- (X = Cl, Br, I). Polonium(IV) oxide is formed by reaction between Po and O₂ at 520 K; it adopts a fluorite lattice  and is sparingly soluble in aqueous alkali. The observed properties are those expected by extrapolation from Te.

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