Selenium criteria

Based on these criteria, selenium compounds can formally be divided into the following classes ... 

The realization that the better ordering criterion is atomic number rather than atomic weight invites us to consider triads of atomic numbers. This reveals a most remarkable fact, namely that —50% of all conceivable triads on a conventional periodic table are in fact exact. For example, the elements sulfur, selenium, and tellurium have atomic numbers of 16, 34, and 52, respectively, thus showing that the atomic number of the middle of these three... 

The magnetic criterion is particularly valuable because it provides a basis for differentiating sharply between essentially ionic and essentially electron-pair bonds Experimental data have as yet been obtained for only a few of the interesting compounds, but these indicate that oxides and fluorides of most metals are ionic. Electron-pair bonds are formed by most of the transition elements with sulfur, selenium, tellurium, phosphorus, arsenic and antimony, as in the sulfide minerals (pyrite, molybdenite, skutterudite, etc.). The halogens other than fluorine form electron-pair bonds with metals of the palladium and platinum groups and sometimes, but not always, with iron-group metals. 

An analogous behavior is found for the other elements, e.g., phosphorus, arsenic, selenium, etc. Paraelements of this sort are no longer electronically isovalent with their reference elements they do, however, remain isovalent with the reference element. Thus the criterion of isovalence is common to all paraelements. 

WHO (1987) Environmental Health Criterion 58 Selenium. World Health Organization. 

From the viewpoint of chemical and physical requirements the permissible indicators involve a number of criteria mercury, selenium, cadmium, vanadium, chromium, arsenic, silver, lead, barium, cyanides, hydrogen sulphide, fluorides, phenols, oil and oil substances, COD(Mn), colour, odour, taste and turbidity. The most stringent criterion concerns the content of mercury, followed by selenium, cadmium, vanadium, cyanides, oil and oil substances. Another important indicator is the content of organic matters characterized by COD(Mn). 

Since the extravalent d orbitals of sulfur, selenium, and tellurium are not occupied in the normal states of the free atom, their implicit functional form is determined by another criterion. The values of a number of molecular properties including ionization potentials, energies of electronic transitions, and overlap populations have been examined as a... 

Selenium may be a problem in the freshwater environment 5 pg/1 is the criterion for protection of aquatic life at chronic exposure and modifications of this criterion have been proposed recently [90]. A similar situation apparently does not exist in the marine environment. Selenium is a relatively abundant trace element in the marine environment and seems to counteract the toxic effects of mercury. 

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