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What is an Oxidation Number?

An oxidation number is also known as an oxidation state. This is an value assigned to an atom which indicates the electron state. Positive oxidation numbers indicate that the atom is lacking electrons. Negative oxidation number distinguishes having extra electrons. Tracking the oxidation number of an atom in a reaction is useful to determine whether oxidation (loss of electrons) or reductions (gain of electrons) has occurred.

Rules for Assigning Oxidation Number

Assigning oxidation numbers to each atom is important when considering the movement of electrons in a reaction. The rules to follow are:

1. An oxidation number of 0 is assigned when an atom in its original or elemental state. Examples: $H_2$, $Cl_2$, $Ca$, $N_2$, $He$.

1. Atoms further left in the periodic table have positive oxidation numbers. Atoms further right in the periodic table have negative oxidation numbers

1. Charge is equal to oxidation state when the atom is by itself (monoatomic). Examples: $K^+$= +1 oxidation, $Mg^{+2}$= +2 oxidation, $Cl^-$= -1 oxidation, $Fe^{3+}$= +3 oxidation.

1. Atoms in polyatomic molecules or ions typically have the same oxidation number as their monoatomic state. Examples: $H_2O$ (H=+1, O=-2), $NH_3$ (N=-3, H=+1).

1. The oxidation states of atoms in a polyatomic molecule must add up to the polyatomic charge. Example: $(SO_4)^{2-}$, O= -2, S= +6.

1. Oxygen typically has an oxidation of -2. An exception to this is in peroxides where oxygen atoms bound to each other, each atom has a -1 oxidation number.

1. Group 1 atoms have +1 oxidation states and group 2 atoms have +2 oxidation states. Halogens have an oxidations of -1.

1. Hydrogen has an oxidation state of +1 or -1. In metal hydrides like $NaH$, the hydrogen is -1. Hydrogen bound to atoms on the right side of the periodic table is +1.

Assigning Oxidation Numbers Examples

Let's work through some examples to assign oxidation numbers!

$KMnO4$ is a chemical compound. Permanganate, $(KMnO4)^{-1}$, is common a polyatomic anion. The K atom will have a charge of +1 per rule 7 above. Considering rule 5, the oxidation numbers in the permanganate anion must be equal to -1. Oxygen atoms are not bound together and will be -2 each (rule 6). If each oxygen is -2, combined they are -8. To generate a -1 charge overall, manganese must have an oxidation state of +7 to create the polyatomic ion charge. In summary: K=+1, Mn=+7, O=-2.

In this reaction, we'll determine the oxidation numbers, oxidized atom, and reduced atom: $$Al\ +\ Fe_2O_3\ \rightarrow\ 2Fe\ +\ Al_2O_3$$ Starting with the reactants Al can be assigned an oxidation of 0 (rule 1). The oxygen atoms in iron oxide will be -2 each and -6 overall. The iron oxide neutral so iron atoms must be +3 and +6 overall (rule 5).

Fe atoms in products have oxidation numbers of 0. In aluminum oxide the oxygens are -2 and -6 overall, which makes Al atoms equal to +3.

Now if we consider the atom oxidation number changes we can assign redox species. Fe atoms gain electrons in the product, making Fe the reduced species. Al loses electrons in the product, making Al the oxidized species.

(Video) 19.1 How to Assign Oxidation Numbers | General Chemistry

by Chad's Prep

In this video, Chad shows you how to assign oxidation numbers. He has six rules. If you work your way through the rules, you get the right answer every time. He illustrates his rules with many unique example problems. Are you struggling with determining oxidation states? If so, this video can help!