A HANDY SYSTEM FOR DETERMINING OXIDATION NUMBERS

TO DETERMINE THE OXIDATION NUMBER OF AN ATOM

Apply the rules below in the order given until oxidation numbers for all but one kind of atom have been assigned.

Use Note A to determine the oxidation number of the final element.

Occasionally application of the six rules leaves two or more kinds of atoms unassigned. If this should occur, choose the most electronegative atom of those still unassigned and assign it the minimum allowable oxidation number (see Note B). Continue this process until all but one kind of atom has been assigned, then apply Note A.
If the substance contains a polyatomic ion, be sure that the oxidation numbers assigned result in the correct charge for that ion. Conversely, the known charge on a polyatomic ion can be used to assign the proper oxidation numbers.
Check each assignment to see that it does not violate Notes B or C.

RULES

  1. The oxidation number of a neutral element is zero.
  2. Fluorine always has an oxidation number of -1 in compounds.
  3. The elements of groups IA (e.g., Na, K), IIA (e.g., Mg, Ca), and IIIA (e.g., Al, Ga) always have positive oxidation numbers of +1, +2, and +3, respectively, in compounds.
  4. Hydrogen has an oxidation number of +1 in all its compounds except in binary and ternary compounds where the only other atoms are metals or boron.
  5. Oxygen has an oxidation number of -2 in compounds, except for some compounds when combined with F, to which rules 2-4 apply.  Oxygen, as the peroxide ion (O22-), has an oxidation number of -1.
  6. The elements of groups VA, VIA, and VIIA have oxidation numbers of -3, -2, and -1, respectively, when found in binary compounds with metals or hydrogen.

NOTES

  1. The sum of the oxidation numbers for the atoms in any species must equal the electrical charge on the species.
  2. The minimum oxidation number of an element is equal to the group number (from the periodic chart) minus 8.
  3. The maximum oxidation number of an element (except elements in group IB) is equal to the group number from the periodic chart.

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Solve Oxidation Number Problems: Oxidation Number Problems