Chalcogens, group trends

The group 16 elements or chalcogens (which we will occasionally abbreviate as Ch) exhibit considerable chemical diversity, with the greatest discontinuity in properties between oxygen and sulfur. Sulfur and selenium are chemically rather similar. Tellurium is considered a metalloid and polonium a full-fledged metal. Some general group trends are as follows. 

Unlike the aryl methyl selenides, the alkyl aryl selenides with alkyl groups of two carbons or more are able to undergo scission of the radical cation at the Cgp-Se bond, yielding the ArSe radical (Fig. 35). This can impact product distribution, particularly under conditions in which water is not present to react with the radical cation. Trends in ease of oxidation typically seen in series of chalcogen compounds can still be observed, and are consistent with trends in the diaryl chalcogenides and aryl methyl chalcogenides. 

Their hydrides MH3, and trihalides are pyramidal but pentahalides are trigonal bipyramidal Group-16 Elements Chalcogens Increasing Decreasing Exceptions Trends Trends... 

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