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1. The oxidation number of any element in its elemental form is zero.
2. The oxidation number of any monoatomic ion is the same as its charge
3. All Group 1 elements (Alkali metals), except hydrogen, have an oxidation number of +1.
4. All Group 2 elements (Alkaline Earth metals) have an oxidation number of +2.
5. Fluorine always has an oxidation number of -1 in its compounds.
6. Oxygen has an oxidation number of -2 except as a peroxide (i.e. sodium peroxide, Na2O2)
7. Hydrogen has an oxidation number of +1 except in metal hydrides (i.e. sodium hydride, NaH)
8. The more electronegative elements always have the negative oxidation number and use the column "charge" as the number value (i.e. CO, O-2, C+2 since O is more electronegative than C)
9. The algebraic sum of all the oxidation numbers of all the atoms in a neutral compound is zero.
10. For polyatomic ions, the algebraic sum of the oxidation numbers of all the atoms in the ion must equal the ion's charge.
Example - What is the oxidation number of C and N in cyanide ion?
a. Use Rule #8 and #10
b. N is more electronegative than C so N has the negative oxidation number.
c. N is in the third column over from Noble Gas so it has an oxidation number of -3
d. Using Rule 10, C must be +2.