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Determining Oxidation Numbers 1. For an atom in its elemental form, the oxidation number is always zero. An example is H2. Another is P4. 2. For any monatomic ion, the oxidation number equals the charge on the ion. So, K+ is +1, S2- is -2. On chart, group IA, alkali metals, is always +1, group IIA, alkaline earth metals, is always +2, and aluminum, group IIIA, is always +3. 3. Nonmetals usually have negative oxidation numbers, but they can be positive: a. Oxygen is usually -2, the major exception being peroxides, which contain the O22- ion. b. Hydrogen is +1 when bonded to nonmetals, -1 when bonded to metals. c. Florine is always -1. The other halogens are also -1 in most binary compounds. The exception is when bonded to oxygen, as in the oxyanions, where they have positive charges. 4. The sum of all oxidation numbers of all atoms in a compound is equal to the charge of that compound. |
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