= Oxidation state
{title2=$\mathrm{OS}$}
{wiki}
= Oxidation number
{synonym}
An <oxidation state> is an <electron>-bookkeeping assignment to a <chemical atom>: its <electric charge> number after the <ionic approximation of bonds>. It helps identify <oxidation> and <reduction> and determine <stoichiometry>, even for <covalent bonds> without separate charged <ions>. In <water>, each <hydrogen> has <oxidation state> $+1$ and <oxygen> has $-2$; this does not assert that a <water> <molecule> contains two free <protons> and a free oxide <ion>.<Comprehensive definition of oxidation state (IUPAC Recommendations 2016) by Pavel Karen and others>[Karen, McArdle and Takats, Comprehensive definition of oxidation state (IUPAC Recommendations 2016), §2, pp. 832–833: definition and equal splitting of same-element bonds]{ref}
Three conventions answer different questions. <Formal charge> divides each bonding <electron pair> equally in a chosen <Lewis structure>. <Oxidation state> uses the <ionic approximation of bonds>. <Partial atomic charge> instead partitions the actual <electron density> and depends on that partition. In the ordinary <water> <Lewis structure>, every <formal charge> is zero although the <oxidation states> are not. Neither bookkeeping convention supplies numerical <partial atomic charges>.
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