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Sodium loses an electron - oxidation 2Na Cl2 2Na Cl Chlorine gains an electron - reduction Magnesium loses an electron - oxidation 2Mg O2 2Mg O 2 Oxygen gains an electron - reduction 2 Oxidation Numbers • In order to keep track of electrons during chemical reactions, the idea of oxidation numbers (oxidation states) was introduced. Oxidation Number • Is the charge which an atom has, or appears to have, in a compound. Rules for Assigning Oxidation Numbers 1. The oxidation number of any uncombined element is zero. Na O2 Cl2 Fe 2. The oxidation number of an ion is the same as its charge. O -2 Fe has an oxidation number of 2 2 has an oxidation number of 2 Na has an oxidation number of 1 3. The sum of all the Oxidation numbers in a compound must equal zero. 4. Oxygen always has an oxidation number of -2 except in peroxides ( when it has an oxidation number of -1) and in the compound OF2 (where it has an oxidation number +2.) 5. Hydrogen is always +1 except in metal hydrides where it is -1. 6. In simple compounds, Halogens have an oxidation number of -1. 7. In a complex ion, the sum of the oxidation numbers should equal the charge on the ion. 1. The oxidation number of any uncombined element is zero. 2. The oxidation number of an ion is the same as its charge. 3. The sum of all the Oxidation numbers in a compound must equal zero. 4. Oxygen always has an oxidation number of -2 except in peroxides ( when it has an oxidation number of -1) and in the compound OF2 (where it has an oxidation number +2.) 5. Hydrogen is always +1 except in metal hydrides where it is -1. 6. In simple compounds, Halogens have an oxidation number of -1. 7. In a complex ion, the sum of the oxidation numbers should equal the charge on the ion. Loses 2e MnO 2H 2Cl Mn +7 4 -8 +2 -2 Cl2 H2 O 2 +2 0 +2 -2 Gains 5e Loses 10e 2MnO 2H 10Cl 2Mn 4 Gains 10e 2 5Cl2 H2 O Loses 10e 2MnO 16H 10Cl 2Mn 4 Gains 10e 2 5Cl2 8H2O Loses 2e MnO H2 S 2H Mn 4 +7 -8 +2 -2 Gains 5e +2 +2 2 S H2 O 0 +2 -2 Loses 2e 0 +4 +6 -2 Gains 2electrons Reduction Loses 2e 2 0 +4 -6 +2-2 Reduced Gains 2 electrons -2 +6 -8 +1