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Assigning Oxidation Numbers to Elements in a Chemical Formula

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  • 0:07 Oxidation Numbers
  • 2:09 Oxidation Number Rules
  • 3:42 Examples
  • 6:14 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn the importance of oxidation in chemical reactions. Discover the rules for assigning oxidation numbers in both covalent compounds and ionic compounds. Learn how to assign the oxidation number for each element in a chemical formula.

Oxidation Numbers

Have you ever taken a pottery class or wondered how a potter gets such pretty glazes on their art? Potters apply a glaze containing many elements - often transition metals - to their unfinished pieces of work. This glaze goes on dull, and it is usually a pastel color, but the finished product comes out of the kiln (or oven) with bright shiny colors. This is because the metals in the glaze are oxidized, and this chemical reaction causes the metal to change colors. The same glaze baked at different temperatures can even be different colors. A glaze that contains iron can form iron (III) oxide, which is red, when baked at a high temperature, or it can form iron (II) oxide, which is black, when baked at a lower temperature.

This lesson will cover the state of oxidation. You've previously learned about reduction-oxidation reactions, known as redox reactions, where one reactant loses electrons, becoming oxidized, and the other reactant gains electrons, becoming reduced. You have also learned what an ion is and how it has an ionic charge that shows the distribution of electrons in an ionic compound. An oxidation number is a number assigned to atoms in molecules to show the general distribution of the electrons. Oxidation numbers assign ownership of the electrons to one atom or another in a compound. However, these numbers don't actually have physical meaning. Although we assign an electron as belonging to an atom, it doesn't actually belong there in real life.

Scientists use these numbers to help name compounds, write formulas and balance chemical equations. They are also useful when studying reactions and they can help you identify when something is oxidized or reduced. If the oxidation number decreases, the atom was reduced. If the oxidation number increases, the atom was oxidized. Oxidation numbers can also help identify differences in an element, which may behave differently in different compounds.

Oxidation numbers help define if an atom was reduced or oxidized.
Numbers Reduced Oxidized

Oxidation Number Rules

There is a set of rules that can be followed for atoms in order to assign oxidation numbers.

  1. In general, if electrons are shared, they belong to the more electronegative atom in each bond.
  2. Atoms of pure elements have an oxidation number of zero. So, pure sodium (Na), oxygen (O2) or sulfur (S8) all have zero oxidation numbers.
  3. If you have a binary compound, the more electronegative element receives an oxidation number that equals the negative charge it would have if it were an anion. The less electronegative element receives an oxidation number that would equal its charge as a cation.
  4. Fluorine always has an oxidation number of -1.
  5. Oxygen almost always has an oxidation number of -2, except in H2O2 (when it is -1) and in OF2 (when it is +2).
  6. Hydrogen always has an oxidation number of +1 in compounds which contain elements that are more electronegative than it is. Hydrogen has an oxidation number of -1 when combined with metals.
  7. The sum of oxidation numbers of all atoms in a neutral compound is zero.
  8. The sum of oxidation numbers of all atoms in a polyatomic ion is equal to the charge on the ion.
  9. The oxidation number of a monatomic ion is equal to the charge on the ion.

Examples

Let's assign an oxidation number to the compound UF6. Look at the rules. From rule number four, you know that fluorine (F) has an oxidation number of -1. That number goes above the F. So, it looks like this:

In this compound, fluorine has an oxidation number of -1.
fluorine oxidation number

You know that in this compound there are six fluorine atoms, so multiply the -1 times 6 for -6. This number goes below the F. From rule number seven, you know that the sum of oxidation numbers of all atoms in a neutral compound is zero. You have figured out that the oxidation number on F6 is -6, so to get zero, U has to have a charge of +6. +6 + (-6) = 0. The whole thing looks like this:

This is a neutral compound, so the sum of the oxidation numbers is zero.
uf6 oxidation number

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