Page 213 - Theory and Problems of BEGINNING CHEMISTRY

202 OXIDATION AND REDUCTION [CHAP. 14

C Each O atom
Number of valence electrons in free atom 4 6
− Number of valence electrons “controlled” −0 −8
Oxidation number +4 −2

Like the charge on an ion, each atom is assigned an oxidation number. Do not say that oxygen in CO 2 has
an oxidation number of −4 because the two oxygen atoms together control four more electrons in the compound
than they would in the free atoms. Each oxygen atom has an oxidation number of −2.
Although the assignment of control of electrons is somewhat arbitrary, the total number of electrons is
accurately counted, which leads to a main principle of oxidation numbers:

The total of the oxidation numbers of all the atoms (not just all the elements) is equal to the net charge
on the molecule or ion.
For example, the total of the three oxidation numbers in CO 2 is 4 + 2(−2) = 0, and the charge on CO 2 is 0.
One principal source of student errors is due to confusion between the charge and oxidation number. Do
not confuse them. In naming compounds or ions, use Roman numerals to represent positive oxidation numbers.
(The Romans did not have negative numbers.) In this book, charges have the numeral first, followed by the
sign; oxidation numbers have the sign first, followed by the numeral. In formulas, Arabic numeral superscripts
represent charges. While working out answers, you might want to write oxidation numbers encircled and under
the symbol for the element. Individual covalently bonded atoms do not have easily calculated charges, but they
do have oxidation numbers. For example, the oxidation number of each element and the charge on the ion for
2− −
SO 3 and for Cl are shown below.

Charges
2− −
SO 3 Cl
Oxidation numbers: +4 −2 −1

It is too time consuming to calculate oxidation numbers by drawing electron dot diagrams each time. We
can speed up the process by learning the following simple rules:
1. The sum of all the oxidation numbers in a species is equal to the charge on the species.
2. The oxidation number of uncombined elements is equal to 0.
3. The oxidation number of every monatomic ion is equal to its charge.
4. In its compounds, the oxidation number of every alkali metal and alkaline earth metal is equal to its group
number.
5. The oxidation number of hydrogen in compounds is +1 except when the hydrogen is combined with active
metals; then it is −1.
6. The oxidation number of oxygen in its compounds is −2, with some much less important exceptions. The
1
oxidation number of oxygen in peroxides is −1, in superoxides it is − , and in OF 2 and O 2 F 2 it is positive.
2
The peroxides and superoxides generally occur only with other elements in their maximum oxidation states.
You will be able to recognize peroxides or superoxides by the presence of pairs of oxygen atoms and by the
fact that if the compounds were normal oxides, the other element present would have too high an oxidation
number (Sec. 14.3).

sodium peroxide oxidation number of sodium = 1
Na 2 O 2
barium peroxide oxidation number of barium = 2
BaO 2
tin(IV) oxide oxidation number of tin = 4 (permitted)
SnO 2
potassium superoxide oxidation number of potassium = 1
KO 2
hydrogen peroxide oxidation number of hydrogen = 1
H 2 O 2