Rules For Assigning Oxidation Numbers
1. For an atom is elemental form, the oxidation number is always zero.
2. For any monatomic ion the oxidation number equals the ionic charge.
In ionic compounds:
-Alkali metal ions (group 1A) always have a +1 oxidation number.
-Alkaline metals (group 2A) have an oxidation number of +2.
-Aluminum has an oxidation number of +3.
3. Nonmetals usually always have negative oxidation numbers equivalent to their
charges with a few exceptions:
a. Oxygen has oxidation number of -1 when found in peroxide but -2 all other
times.
b. Hydrogen will always have a +1 oxidation number unless it is bonded with
another metal, then its oxidation number is -1.
c. Fluorine will ALWAYS have an oxidation number of -1. The other halogens
will always have on oxidation number of -1 in binary compounds except for when
they are bonded with oxygen. Then they may have a positive oxidation number.
4. The sum of the oxidation numbers of all atoms in a neutral compound is zero. The
sum of the oxidation numbers in a polyatomic ion equals the charge of the ion.