Oxidation States

The oxidation state (or oxidation number) is a number that is applied to each atom in a compound in such a way as to keep track of changes occurring in redox reactions. For easy ionic compounds the oxidation state is equivalent to the charge on the ions. Roman numbers should be employed to differentiate oxidation states from ionic charges, example NaI and Cl-I in NaCl. The electrons are assigned to the more electronegative atoms as if the bonding were ionic, in polar covalent bonds. Oxidation states are measured by the following rules.

(i) Bonds between similar elements are not counted. The Elements have oxidation state zero. In an ion like peroxide the electrons in the O-O bond are distributed similarly, making O^-I.

(ii) Except in cases like the most electronegative and electropositive elements in a compound have an oxidation state identical to their normal ionic charge: KI, Ca^II, F^-I, O^-II.

(iii) The sum of the oxidation states must equivalent to the charge on the species and so is zero in a neutral compound. By Using this rule and (ii) above, we have HI in H₂O, H-I in CaH₂ and Mn^VII in .

(iv) Complex formation and donor-acceptor interaction generally do not change the oxidation state. Both [Ni(NH₃)₆]²⁺ and [Ni(CN)₄]^2- have Ni^II, complexed by NH₃ and CN^- respectively.

A redox reaction is any reaction that is involving changes of oxidation state. In Equation no.1 the changes are from Zn⁰ to Zn^II and from H^I to H⁰. The reaction is:

ZnO(s) + 2H^{+ (}aq) → Zn²⁺ (aq)  + H₂O

is not a redox reaction as no alter of oxidation state occur.

Identifying the oxidation state of an element can be a helpful method of naming compounds, particularly when variable stoichiometries are possible. So we have iron(II) chloride (FeCl₂) and iron(III) chloride (FeCl₃). The older names are 'ferrous' and 'ferric' correspondingly are still encountered for such type of compounds but are potentially confusing.

In present terminology the -ous and -ic suffixes (considering to a lower and a higher oxidation state, correspondingly) are only employed for some oxoacids (example H₂SIVO₃, sulfurous acid and H₂SVIO₄, sulfuric acid;).