Stock Notation

Stock notation, named after the German chemist Alfred Stock, is a compact way of showing an element's oxidation number directly inside a compound's formula: the oxidation number is written as a Roman numeral inside round brackets, placed immediately after the symbol of the element it describes. For example, AuCl (gold at +1) is written Au(I)Cl; AuCl3 (gold at +3) is written Au(III)Cl3; SnCl4 (tin at +4) is written Sn(IV)Cl4; SnCl2 (tin at +2) is written Sn(II)Cl2; and MnO2 (manganese at +4) is written Mn(IV)O2. Stock notation is used specifically to pin the oxidation state of a metal (rather than a non-metal), because it is the metal's oxidation number that most often needs to be flagged unambiguously -- many metals, unlike fluorine or the alkali metals, can adopt more than one oxidation state across their different compounds (iron as Fe(II) or Fe(III), tin as Sn(II) or Sn(IV), and so on), and Stock notation removes any doubt about which one a given formula refers to. This idea of oxidation number, and its Stock-notation shorthand, is what makes it possible to define oxidation, reduction, oxidising agent a …