1 Assigning oxidation number

Rules for assigning oxidation numbers 1. All uncombined elements have an oxidation number of zero eg . Zn, Cl2, O2, Ar all have oxidation numbers of zero 2. The oxidation numbers of the elements in a compound add up to zero In NaCl Na= +1 Cl= -1 Sum = +1 -1 = 0 3. The oxidation number of a monoatomic ion is equal to the ionic charge e.g. Zn2+ = +2 Cl- = -1 4. In a polyatomic ion (CO3 2-) the sum of the individual oxidation numbers of the elements adds up to the charge on the ion e.g. in CO3 2- C = +4 and O = -2 sum = +4 + (3 x -2) = -2 5. Several elements have invariable oxidation numbers in their common compounds. Group 1 metals = +1 Group 2 metals = +2 Al = +3 H = +1 (except in metal hydrides where it is –1 eg NaH) F = -1 Cl, Br, I = –1 except in compounds with oxygen and fluorine O = -2 except in peroxides (H2O2 ) where it is –1 and in compounds with fluorine. We use these rules to identify the oxidation numbers of elements that have variable oxidation numbers. Note the oxidation number of Cl in CaCl2 = -1 and not -2 because there are two Cl’s Always work out the oxidation for one atom of the element What is the oxidation number of Fe in FeCl3 Using rule 5, Cl has an O.N. of –1 Using rule 2, the O.N. of the elements must add up to 0 Fe must have an O.N. of +3 in order to cancel out 3 x –1 = -3 of the Cl’s