Oxidation Number Calculation

One of the most necessary procedures in Chemistry is learning how to calculate oxidation numbers. Oxidation states are straightforward to work out and to use, but it is quite difficult to define what they actually are in any quick way. How do we calculate oxidation numbers?

First off, an oxidation number is the the degree of oxidation of an atom, ion, or molecule; for simple atoms or ions the oxidation number is equal to the ionic charge.

For example, the oxidation number of hydrogen is +1 and of oxygen is -2.

It helps to use a periodic table to determine oxidation numbers.


In between +2 and +3 we do not assign any numbers because there tends to be more than one oxidation number assigned to those elements.

Although the above method is dependable, oxidation states change. Elements can be oxidized or reduced.

Oxidation involves an increase in oxidation state or the decrease in number of electrons

Reduction involves a decrease in oxidation state or the increase in number of electrons

Recognising this simple pattern is the single most important thing about the concept of oxidation states. A few rules to follow when looking for the oxidation number include:

  • The oxidation state of an uncombined element is zero. That’s obviously so, because it hasn’t been either oxidised or reduced yet! This applies whatever the structure of the element – whether it is, for example, Xe or Cl2 or S8, or whether it has a giant structure like carbon or silicon.
  • The sum of the oxidation states of all the atoms or ions in a neutral compound is zero.
  • The sum of the oxidation states of all the atoms in an ion is equal to the charge on the ion.
  • The more electronegative element in a substance is given a negative oxidation state. The less electronegative one is given a positive oxidation state. Remember that fluorine is the most electronegative element with oxygen second.
  • Some elements almost always have the same oxidation states in their compounds but some, like Hydrogen although usually +1, can be different.

A few examples that you can work out are below.

What is the oxidation state of chromium in Cr2+?

For a simple ion like this, the oxidation state is the charge on the ion – in other words: +2 (Don’t forget the + sign.)

What is the oxidation state of chromium in CrCl3?

This is a neutral compound so the sum of the oxidation states is zero. Chlorine has an oxidation state of -1. If the oxidation state of chromium is n:

                                                n + 3(-1) = 0

                                                n = +3 (Again, don’t forget the + sign!)

I hope this helps you as much as it has helped me.

-Nnana Amakiri




5 thoughts on “Oxidation Number Calculation

  1. Excellent job Namakiri! There is another important way to calculate the oxidation state of an atom in a molecule. If you have the Lewis structures of the molecules and we know the electronegativity of the bonded atoms, we can assign them using a “bond cutting approach.” Check out this video if interested: http://www.youtube.com/watch?v=N2zXn9-GjYU

    PS – Interesting mnemonic PT IZ Michopoulos: “OIL RIG” should be easy to remember! I hadn’t heard that before.

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