Codex @codex  0

 Incoming links: Comprehensive definition of oxidation state (IUPAC Recommendations 2016) by Pavel Karen and others

For the routine examples here, assign all bonding Electrons between different chemical elements to the more electronegative partner; divide bonding Electrons between identical chemical elements equally. Retain each lone pair on its original chemical atom. Subtract the assigned valence electron count

N_{i}^{assigned}

v_{i}

. The IUPAC convention uses Allen electronegativity, based on free-chemical atom valence electron energies. Close electronegativity comparisons and some reversibly bound chemical compounds require care beyond this simple rule.^[ref]

The examples explain exceptions to memorized rules:

Water, $H - O - H$ : oxygen receives both O–H bond pairs plus its two lone pairs, giving eight assigned valence electrons and $6 - 8 = - 2$ . Each hydrogen receives none, giving $1 - 0 = + 1$ .
Hydrogen peroxide, $H - O - O - H$ : each oxygen receives its O–H bond pair, one Electron from O–O, and four Electrons in lone pairs. Thus each has $6 - 7 = - 1$ , while each hydrogen remains $+ 1$ . The O–O connection is why the usual $- 2$ oxygen shortcut fails.
Molecular oxygen, $O_{2}$ : equal splitting of the O–O bonding Electrons leaves each oxygen assigned six valence electrons, so each has oxidation state zero.
Sodium hydride, $NaH$ : hydrogen is more electronegative than sodium. The formula unit is assigned sodium $+ 1$ and hydrogen $- 1$ , consistent with the hydrogen anion. The usual $+ 1$ hydrogen shortcut fails here.

These drawings record allocations, not molecular geometry or a physical process of breaking the chemical bonds. In particular, the O–O line in the molecular oxygen panel only marks the connection; it does not assert a complete Lewis structure or spin state.

+ 1

and oxygen has

- 2

; this does not assert that a water molecule contains two free protons and a free oxide ion.^[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.