Page 212 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 14

Oxidation

and Reduction

14.1. INTRODUCTION
We learned to write formulas of ionic compounds in Chaps. 5 and 6. We balanced the charges to determine
the number of each ion to use in the formula. We could not do the same thing for atoms of elements in covalent
compounds, because in these compounds the atoms do not have charges. In order to overcome this difficulty, we
define oxidation numbers, also called oxidation states.
In Sec. 14.2 we will learn to determine oxidation numbers from the formulas of compounds and ions. We
will learn how to assign oxidation numbers from electron dot diagrams and more quickly from a short set of
rules. In Sec. 14.3 we will learn to predict oxidation numbers for the elements from their positions in the periodic
table in order to predict formulas for their compounds and ions. We use these oxidation numbers for naming
the compounds or ions (Chap. 6 and Sec. 14.4) and to balance equations for oxidation-reduction reactions (Sec.
14.5). In addition, we will use oxidation-reduction reactions to discuss electrochemical reactions in Sec. 14.5.

14.2. ASSIGNING OXIDATION NUMBERS
In Sec. 5.5, electron dot diagrams were introduced. The electrons shared between atoms were counted as
“belonging” to both atoms. We thus counted more valence electrons than we actually had. For oxidation numbers,
however, we can count each electron only once. If electrons are shared, we arbitrarily assign “control” of them
to the more electronegative atom. For atoms of the same element, each atom is assigned one-half of the shared
electrons. The oxidation number is then defined as the number of valence electrons in the free atom minus the
number “controlled” by the atom in the compound. If we actually transfer the electrons from one atom to another,
the oxidation number equals the resulting charge. If we share the electrons, the oxidation number does not equal
the charge; there may be no charge. In this case, control is not meant literally, but is just a term to describe the
counting procedure. For example, the electron dot diagram of CO 2 may be written as

O C O
Since O is to the right of C in the second period of the periodic table, O is more electronegative, and we assign
control of all eight shared electrons to the two O atoms. (It does not really have complete control of the electrons;
if it did, the compound would be ionic.) Thus, the oxidation number of each atom is calculated as follows:

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