Oxidation Numbers

Reviewing the oxidation numbers is helpful in determining the degree of oxidation or reduction. Oxidation numbers are also referred to as oxidation states. The number is simply the charge of an ion in the case of ionic compounds. Oxidation numbers are assigned to atoms within molecular compounds or atoms within polyatomic ions. In these cases, the oxidation number can be thought of as similar to a charge or like a charge. However, it is not an actually charge. Why? Atoms are not ions. Common elements like sodium, lithium, and calcium have charges of plus one (Na+1, Li+1), and plus two (Ca+2). Elements within a molecular compound can be negative as well. In phosphide and chloride, the potassium and chlorine have a negative oxidation state (P-3 ,Cl-1). Oxygen has a negative two oxidation state in oxide (O-2).

Oxidation StatesThe periodic table provides you will clues about each elements oxidation number. Most metals have an oxidation number of plus one or plus two. Transitions metals oxidation states can range from plus zero to plus six. Noble gases hardly ever react with other element, giving them an oxidation state of zero. The oxidation state is determined by the number of valence electrons that each element contains. In this activity, click on each of the tabs to learn how the oxidation states changes for each group on the periodic table.

View a printable document of the interactivity.

 

Oxidation State Rules

All compounds, whether covalent, polar covalent, or ionic, are treated as though they are ionic for the purpose of counting electrons and for determining oxidation numbers. There are several rules used to identify the oxidation number of elements and compounds. Take a moment to review the rules of oxidation numbers.

  1. The oxidation number of a pure element or an atom in an uncombined state, whether monatomic or polyatomic, is zero. For example, the oxidation number of each atom in Fe, N2, Br, and H4 is zero.
  2. The oxidation number of a monatomic ion is the same as the charge for the ion. For example, the oxidation numbers of K+, Se2−, and Au3+ are plus one, minus two, and plus three. The oxidation number of oxygen in most compounds is minus two. Exceptions to this rule do exist. If oxygen bonds with an element that is more electronegative then its oxidation state will change. Another exception occurred when two oxygen atoms bond together.
  3. The oxidation number of hydrogen in most compounds is plus one. However, if hydrogen bonds with a metal hydride it will carry a charge of minus one.
  4. The oxidation number of fluorine in all compounds is minus one. Other halogens have oxidation numbers that vary depending on the type of bonding.
  5. In a neutral molecule, the sum of the oxidation numbers of all atoms is zero.
  6. In a polyatomic ion, the sum of the oxidation numbers of all atoms is equal to the overall charge on the ion.

Calculating Oxidation States

Calculating Oxidation StatesHow do the oxidation numbers help determine what has been oxidized or reduced? An increase in the oxidation number indicates that an atom has lost electrons and is oxidized. A decrease in the oxidation number indicates that an atom has gained electrons and is reduced. View this presentation to learn how the oxidation number of a compound is calculated by reviewing examples.

View a printable version of the interactivity.

 

Oxidation Numbers Review

Oxidation ReviewSelf Check IconNow that you have learned to calculate the oxidation numbers of different substances, complete this non-graded activity to check your knowledge. Calculate the oxidation numbers for the given molecules. Then, select the appropriate answer and click SUBMIT to check your responses. Click on the interactivity thumbnail, and then click NEXT to get started.