The Rules of Assigning Oxidation Numbers
1. Free elements are assigned an oxidation state of 0.
e.g. Al, Na, Fe, H2, O2, N2, Cl2 etc have zero oxidation states.
2. The oxidation state for any simple one-atom ion is equal to its charge.
e.g. the oxidation state of Na+ is +1, Be2+, +2, and of F-, -1.
3. The alkali metals (Li, Na, K, Rb, Cs and Fr) in compounds are always assigned an oxidation state of +1.
e.g. in LiOH (Li, +1), in Na2SO4(Na, +1).
4. Fluorine in compounds is always assigned an oxidation state of -1.
e.g. in HF2-, BF2-.
5. The alkaline earth metals (Be, Mg, Ca, Sr, Ba, and Ra) and also Zn and Cd in compounds are always assigned an oxidation state of +2. Similarly, Al & Ga are always +3.
e.g. in CaSO4(Ca, +2), AlCl3 (Al, +3).
6. Hydrogen in compounds is assigned an oxidation state of +1. Exception - Hydrides, e.g. LiH (H=-1).
e.g. in H2SO4 (H, +1).
7. Oxygen in compounds is assigned an oxidation state of -2. Exception - Peroxide, e.g. H2O2 (O = -1).
e.g. in H3PO4 (O, -2).
8. The sum of the oxidation states of all the atoms in a species must be equal to net charge on the species.
e.g. Net Charge of HClO4 = 0, i.e. [+1(H)+7(Cl)-2*4(O)] = 0
Net Charge of CrO42-=-2,
To solve Cr's oxidation state: x - 4*2(O) = -2, x = +6, so the oxidation state of Cr is +6.