ChemTalk

How to Find Oxidation Number & Oxidation State

oxidation states and numbers

Oxidation number and oxidation state are two very important concepts in chemistry. In this article, we will make them very easy to understand!

Topics Covered in Other Articles:

Cations and Ions
Redox Reactions
Common polyatomic Ions
Electrochemical cells
What are ions?

Oxidation Number – Definition

The oxidation number represents how many electrons an atom has gained or lost in a molecule. Elements have an oxidation state of zero, and atoms in ionic compounds are usually assigned a positive or negative oxidation state. Organic compounds and some covalent compounds do not have oxidation states assigned to the atoms in the compounds.

Put another way, the the oxidation state of an atom is equal to the number of electrons which have been removed from an element (producing a positive oxidation state) or added to an element (producing a negative oxidation state).

If you want to use the IUPAC definition, here it is: “Oxidation state of an atom is the charge of this atom after ionic approximation of its heteronuclear bonds“

Oxidation State – Definition

The oxidation state of an atom is simply it’s oxidation number. There is no difference*.

(*Read the last section of the article to learn about an exception)

How to find the Oxidation Number

By using some simple rules, you can easily determine the oxidation number of every atom in a molecule.

Oxidation Number Rules

Use these rules to determine the oxidation state of every atom in a compound.

  1. Fluorine is always -1.
  2. Oxygen is always -2, except when it is with fluorine, or when it is in a peroxide or peroxy anion like the peroxide ion, in which case it is -1.
  3. The oxidation state of an atom in its elemental state is zero. Forexample, He, O2, S8
  4. In an ionic compound with 2 elements, the metal or the more electronegative atom has a positive oxidation number, and the non-metal or less electronegative atom has a negative oxidation number.
  5. Alkali metals (Group 1 metals) like sodium and potassium have an oxidation state of +1
  6. Alkaline earth metals (Group 2 metals) like calcium and magnesium are +2
  7. Hydrogen is +1 when combined with a non-metal, for example hydrogen chloride HCL. It is -1 when combined with a metal – for example sodium hydride, NaH.
  8. Halogens have a -1 oxidation number when combined with one other element (binary compounds), except for inter-halogen compounds. For example, sodium fluoride NaF, calcium iodide CaI2, chromium (III) chloride CrCl3, tin (II) bromide SnBr2. In each case, the halogen has an oxidation number of -1.
  9. The charge of a monoatomic ion (ion that consists of 1 type of atom) can tell you the oxidation state. For example, the oxidation state of copper in a copper (II) compound is +2. Mercury in the mercurous ion (Hg2+2) is +1 (you have to divide the charge by 2 in this case). Nitrogen is -3 in the nitride ion, N-3
  10. In a polyatomic ion, the sum of the oxidation states of the individual atoms must equal the charge on the ion. If oxygen is present, assume oxygen is -2. We created a video to explain how this is done.

    Rule 10 oxidation number examples:
    For example, in the nitrate ion NO3–, the charge of the ion is -1. Then, using rule 2, we know the charge of the oxygens is 3 x-2 = -6. So nitrogen must have a charge of +5 to make the charge of the ion -1.

    The chlorite ion, ClO2-, has a charge of -1. Therefore, using rule 2, we know the charge of the oxygens is 2 x -2 = -4. So chlorine must have a charge of +3 to make the charge of the chlorite ion -1.
oxidation state examples
Examples of oxidation states in elements and molecules

Elements with the most oxidation states

Vanadium, manganese, and chromium have the greatest variety of stable oxidation states and colors. For example, the cover photo for this article, taken by Wilco Oelen (who was one of the many inspirations for ChemTalk), shows the colors of vanadium compounds in the +2, +3, +4 and +5 oxidation states. Tungsten and molybdenum also have several oxidation states, some of which are less studied than other transition metals.

Fun fact – the highest known oxidation state is reported to be +9 in the tetroxoiridium(IX) cation (IrO4+).

What is the oxidation state & number of sulfur in the Tetrathionate Ion?

In the tetrathionate ion, S4O6-2, two of the sulfurs have oxidation state of 0, and two have an oxidation state of +5. Therefore, the “average” oxidation number of sulfur in the molecule is (0+0+5+5)/4 = 2.5. It is only in these mixed oxidation state compounds that the concept of oxidation number being different than oxidation state may come up