### Cue Column | Note Column
**Free element rule?** | Any element in its free (uncombined) state has oxidation number = 0. e.g., Fe(s), $O_{2}$(g), $Cl_{2}$(g), Na(s) → all = 0.
**F always?** | Fluorine = −1 in ALL compounds (no exception; most electronegative element).
**O exceptions?** | Usually −2. Exceptions: Peroxides ($H_{2}$$O_{2}$, $Na_{2}$$O_{2}$) = −1; $OF_{2}$ = +2; Superoxides (K$O_{2}$) = −1/2.
**H exceptions?** | Usually +1. Exception: Metal hydrides (NaH, Ca$H_{2}$, LiH) = −1.
**Sum rule?** | For neutral molecule: sum = 0. For ion: sum = charge on ion.
**Common examples?** | Cr in $K_{2}Cr_{2}O_{7}$: +6 (2 + 2x − 14 = 0 → x = +6); Mn in $KMnO_{4}$: +7 (1 + x − 8 = 0 → x = +7); S in $SO_{4}^{2-}$: +6; N in $NO_{3}^{-}$: +5; S in $H_{2}$$SO_{3}$: +4.
**Highest oxidation state?** | Usually = group number in periodic table. Cl can be +7 ($Cl_{2}$$O_{7}$), S can be +6 ($H_{2}$$SO_{4}$), Cr can be +6, Mn can be +7.
**Identifying redox?** | If any element changes oxidation number → redox reaction. Oxidized species: ox. no. increases (loses $e^{-}$). Reduced species: ox. no. decreases (gains $e^{-}$).
### Summary
Key exceptions to memorize: F = always −1; O = −1 in peroxides, +2 in $OF_{2}$; H = −1 in metal hydrides. For any compound, assign known elements first, then solve algebraically for unknown. Oxidation = increase in oxidation number = loss of electrons.$
Part of PC-07 — Redox Reactions & Electrochemistry
Oxidation Numbers — Rules and Applications
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