OxidationNumber

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Rules for Assigning Oxidation Number




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?

CN1-

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.






Rules for Assigning Oxidation Number


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?
CN1-
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.



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