Therefore, elemental carbon (graphite or diamond) has an oxidation number of 0, as does an atom in metallic iron, or each of the two hydrogen atoms in the H 2 molecule:Ī single‐atom ion is assigned an oxidation number equal to its electrical charge. First, the oxidation number of each atom in a pure element is defined as zero. The atoms in H 2O are not ions.įour rules apply when assigning oxidation numbers to atoms. The resulting hypothetical electrical charges are the oxidation numbers, which are shown in parentheses to remind you that they are conceptual rather than real. (See Figure 2.)įigure 2. Assignment of oxidation numbers. The general idea is to assign the shared electrons in each bond to the more electronegative element.Īs an example, use the water molecule in a standard Lewis diagram, as shown in Figure 1.īecause oxygen is more electronegative than hydrogen, for the purpose of assigning oxidation numbers, it is assumed that all four electrons in the two covalent bonds are associated completely with the oxygen atom. A useful extension of this concept is to assign hypothetical charges called oxidation numbers to atoms with polar covalent bonds. The existence of ions suggests a transfer of electrons from one atom to another giving rise to the positive and negative charges. Ions have an electrical charge-negative if they have gained electrons or positive if they have lost electrons. Quiz: Introduction to Oxidation-Reduction Reactions.Introduction to Oxidation-Reduction Reactions. Quiz: Heat Capacities and Transformations.Quiz: Introduction to Organic Compounds.
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