Pavel Karen, Patrick McArdle, and Josef Takats
Pure and Applied Chemistry, 2016Volume 88, Issue 8, pp. 831-839
Oxidation state (OS) gives the degree of oxidation of an atom in terms of counting electrons.  It scales trends in redox and acid-base properties, as well as in physical properties such as magnetism, and is a key component when tracking the course of chemical reactions. Thus, the concept of OS is important, and so is an agreed upon definition of what OS is and the algorithmic manner in which it is to be calculated. In the absence of an actual definition, algorithms have thus far been used to define OS.
This recommendation proposes a definition of OS based on ionic approximation of chemical bonds, illustrated on a molecular-orbital scheme and deduced from electronegativity considerations (Allen’s scale). Two algorithms are formulated for the determination of OS in molecules, ions, and extended solids, and are illustrated with examples. Limits, beyond which OS ceases to be well-defined or becomes ambiguous, are exemplified; moving outside these requires additional measurements, round offs, estimates, or plain postulates. Specific uses that justify a nominal OS are also explained.
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