The Difference Between Oxidation State and Oxidation Number

chemical reaction in test tube

GIPhotoStock / Getty Images

Oxidation state and oxidation number are quantities that commonly equal the same value for atoms in a molecule and are often used interchangeably. Most of the time, it doesn't matter if the term oxidation state or oxidation number is used.
There is a slight difference between the two terms.

Oxidation state refers to the degree of oxidation of an atom in a molecule. In other words, the oxidation state is the charge of an atom if all bonds it formed were ionic bonds. Each atom of the molecule will have a distinct oxidation state for that molecule where the sum of all the oxidation states will equal the overall electrical charge of the molecule or ion. Each atom is assigned an oxidation state value based on predetermined rules based on electronegativity and periodic table groups.

If a molecule has a neutral charge, the sum of all oxidation states of its atoms must equal zero. For example, in the molecule FeCl3, each chlorine atom has an oxidation state of -1, while the iron atom has an oxidation state of +3. The three chlorine atoms cancel out the one iron atom, leaving a net charge of 0.
Oxidation numbers are used in coordination complex chemistry. They refer to the charge the central atom would have if all ligands and electron pairs shared with the atom were removed.