Characteristics of Chalcogens
The chalcogens are the name for the Periodic Table group 16 (V1). The group consists of the elements: oxygen, sulfur, selenium, tellurium, and polonium. The name of the group was proposed by Wilhelm Blitz and colleague Werner Fischer of the Institute of Inorganic Chemistry at the University of Hannover, Germany in 1932. The term “chalcogens” was derived from the Greek word chalkos, meaning “ore formers,” since they all are found in copper ores. Their compounds are referred to as “chalcogenides.” These names became popular since they are analogous to the names of group 17(V11), halogens, meaning salt formers, and its compounds, halides. As an element oxygen is a gas while the other group members are solids. Both oxygen and sulfur can be found in pure form. All of the elements occur as ions in metal ores. The elements of this family have isotopes that vary in stability. Polonium is considered a radioactive element and to be handled with care.
The members of this group show increasing metal character as the atomic number increases. Both oxygen and sulfur are nonmetals, while selenium and tellurium are metalloids (both are semiconductors) and polonium is a metal. Bonding characteristics of these elements parallel their metal character. The smaller members of the group show a greater tendency to form covalent bonds than the larger members do. Their electronegativities decrease with increasing atomic number. This trend explains the lessening of ionic character of the compounds of S – Po that are analogous to those of oxygen. It is for this reason, only oxygen can exhibit hydrogen bonding. This drastically changes the character of these compounds. If the boiling points of the H2M compounds are compared, we find that the group shows this change in hydrogen bonding, since the smallest compound in weight has a boiling point almost 100° higher than any of the other group members.
All of the elements can be found naturally as ions, most often as 2− ions (except for Po). Oxidation states of +2, +4, and +6 also can be found for members S – Po when combined with O, F, or Cl. The difference in oxidation states can be explained by the electronic structure of the elements. Oxygen is able to use only s and p orbitals for bonding. The larger members of the group use d orbitals in the hybridization and thus can participate in the use of an expanded octet. This is shown in compounds and complex ions such as: SOF4, SF4, SeO42, TeF82.
The elements and their compounds vary in their toxicity. It is interesting that oxygen and sulfur are essential to all life, while the compounds of selenium, tellurium, and polonium can be toxic. Selenium is an essential trace element even though at larger concentrations (above 5 ppm) it causes severe disorders. Tellurium compounds, while being toxic, have never been reported to cause a human fatality. Since polonium and its compounds are radioactive, they are considered extremely toxic in any concentration.
Dr. Badruddin Khan teaches Chemistry in the University of Kashmir, Srinagar, India.
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- expanded octet for tellurium