Oxoacids of selenium and tellurium

With concentrated nitric acid, selenium and tellurium form only their +4 oxoacids, H2Se03 and H2Te03 respectively, indicating a tendency for the higher oxidation states to become less stable as the atomic number of the element is increased (cf. Group V, Chapter 9). 

The trioxides, SeOg and TeOg, are also known, but they are not obtained by oxidation of the dioxide as is the case with SO3. The selenium and tellurium trioxides are usually prepared by dehydration of their respective oxoacids, H2Se04 and Te(OH)6 or (HgTeOe). 

Table 17.2 shows the formulas of the catenated sulfur oxoacids and one resonance structure for each corresponding anion. Note from the table that not all these acids exist in the free state, but all the anions are well-known. None of these acids or anions have selenium or tellurium analogs in which aU the sulfur atoms are replaced by a heavier congener. 

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