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.