Oxidation numbers for atoms, ions and compounds

The oxidation number of an atom reflects the number of electrons that it has. Although it looks like a charge, it is important to remember that the oxidation number is not the same as the charge on the atom.

The general rules for determining the oxidation number are:

• The oxidation number of a pure element is always 0 (zero).
• The oxidation number for a monoatomic ion is equivalent to its charge.
• In general, the oxidation numbers of hydrogen, oxygen, and fluorine in a compound are +1, -2 and -1, respectively.
• The general oxidation numbers for some other elements, such as metals, can be determined based on their position in the periodic table.
• The sum of the oxidation numbers for all atoms in any chemical species is equal to the charge, i.e for example. 0 for neutral molecules, -1 for anions with a net charge of -1, etc.