COMPOUNDS

5.10 - Oxidation Numbers

In section 4.6, we learned the specific charge of ions formed by representative elements. In section 5.1.1 we learned that when an atom loses an electron to form a cation, the atom is said to be oxidized. When an atom gains an electron to form an anion, the atom is said to be reduced.

We can assign oxidation number to each element in a compound to keep track of electrons in oxidation and reduction processes.
Oxidation numbers can be zero, positive or negative numbers. For this course, you'll only encounter oxidation numbers as integers. Aside from the simple ions in the compound, oxidation numbers bear no real physical meaning.

In polyatomic compounds which contain polyatomic ions*, elements in polyatomic ions do not have measurable ionic charge. For example, in sodium nitrate, NaNO₃,

• the sodium ion carries a +1 charge (section 4.6)
• the nitrate carries a -1 charge (nitrate is a polyatomic ion*)
• the nitrogen in the nitrate ion does not have any ionic charge since it is covalently bonded to oxygen atoms. However, we can determine an "apparent charge" that is associated with nitrogen.

Since we know that a compound must be neutral in charge, we follow a set of rules to assign the oxidation number to nitrogen. We would determine that the apparent charge on nitrogen is +5.

Let's take a look at how I arrived at an oxidation number of +5 for nitrogen on the next page.

* Click on and find the icon to learn the polyatomic ions.

Section 6.3
Oxidation Numbers. Calculating Oxidation Numbers ..p121

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