Oxidation States (and Ionic Charges)

Rules for assigning oxidation numbers:

1. Uncombined elements (e.g. Ne) have an oxidation number of 0.

2. Atoms in molecules bonded to identical atoms (e.g. O₂) also have an oxidation number of 0.

3. A monatomic ion's oxidation number is equal to its charge. E.g., Fe³⁺ has an oxidation number of +3.

4. In a molecular ion, the sum of the oxidation numbers equals the overall charge. E.g., in SO₄^2-, S is +6 and O is -2, summing to -2.

5. In a neutral compound, the sum of the oxidation numbers is 0. E.g., in Al₂O₃, Al is +3 and O is -2, summing to 0.

6. Combined oxygen usually has an oxidation number of -2, except in peroxides, where it is -1, and in OF₂ where it is +2.

7. Combined hydrogen usually has an oxidation number of +1, except in metal hydrides, where it is -1.

In a compound or ion containing 2 non-metals, the negative oxidation number is given to the more electronegative element. Fluorine always has an oxidation number of -1 because it is the most electronegative element.

When elements can exhibit multiple oxidation numbers, Roman numerals are used after the element's name to specify its oxidation number.

Common Oxidation States

• uncombined element = 0

• group 1 = +1

• group 2 = +2

• group 3 = +3

• oxygen (usually) = -2

• oxygen in hydrogen peroxide = -1

• oxygen when bonded only to fluorine = +2

• fluorine = -1

• chlorine (usually) = -1

• chlorine with fluorine or oxygen = positive

• hydrogen (usually) = +1

• hydrogen in metal hydrides = -1

Ion Charges

nitrate = NO₃^-

carbonate = CO₃^2-

sulfate = SO₄^2-

hydroxide = OH^-

ammonium = NH₄⁺

zinc = +2

silver = +1