What is the basic difference between oxidation number, valency and formal charge?
Valence, and its definition, go back to the 19th century and beyond. Valence was defined as the “combining power” of an element. Remember, the term valence precedes the discovery of electrons. The valence of an element referred to the number of other atoms which could be combined with a single atom the element. The use of positive and negative values for valence did not appear until the early 20th century, and gave rise to the term “covalent”. “Oxidation number” is often used interchangeably with “oxidation state” despite the two terms mean different things. The oxidation state indicates the “degree of oxidation”, and represents the hypothetical charge on a (nonexistent) ion in a compound (keeping in mind that there are no 100% ionic bonds, and no actual ions in compounds). Oxidation state (and by default, oxidation number) is the “new valence”, and is used to predict the formulas of compounds and to track changes in redox reactions. You will see that for a given element in a compound,
