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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
195
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 19
5. The oxidation number of oxygen in compounds is always Ϫ2,
except in peroxides, such as hydrogen peroxide (H 2 O 2 ), where it
is Ϫ1. When it is bonded to fluorine, the oxidation number of
oxygen is ϩ2.
6. The oxidation number of hydrogen in most of its compounds is
ϩ1. The exception to this rule is the metal hydrides, such as
LiH, MgH 2 , and AlH 3 . In these compounds, hydrogen has an
oxidation number of Ϫ1.
7. The metals of groups 1 and 2 and aluminum in group 13 form
compounds in which the metal atom always has a positive
oxidation number equal to the number of its valence electrons
(ϩ1, ϩ2, and ϩ3, respectively).
8. The sum of the oxidation numbers in a neutral compound is
zero.
9. The sum of the oxidation numbers of the atoms in a polyatomic
ion is equal to the charge on the ion.
Many elements not specified in the rules above can have different
oxidation numbers in different compounds. The list of rules enables
you to determine these unknown oxidation numbers, as the following example problem demonstrates.
Example Problem 19-1
Determining Oxidation Numbers
Determine the oxidation number of each element in the following
compound and ion.
a. SrCO 3 (strontium carbonate)
b. Cr 2 O 7
2Ϫ (dichromate ion)
Assign the known oxidation numbers to their elements, set the sum
of all oxidation numbers to zero or to the ion charge, and solve for
the unknown oxidation number (n element ).
a. According to rule 8, the sum of the oxidation numbers is zero
because strontium carbonate is a neutral compound. Rule 5
states that the oxidation number of oxygen in compounds is
Ϫ2, and rule 7 states that group 2 metals, such as strontium,
have a ϩ2 oxidation number in compounds.
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
195
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 19
5. The oxidation number of oxygen in compounds is always Ϫ2,
except in peroxides, such as hydrogen peroxide (H 2 O 2 ), where it
is Ϫ1. When it is bonded to fluorine, the oxidation number of
oxygen is ϩ2.
6. The oxidation number of hydrogen in most of its compounds is
ϩ1. The exception to this rule is the metal hydrides, such as
LiH, MgH 2 , and AlH 3 . In these compounds, hydrogen has an
oxidation number of Ϫ1.
7. The metals of groups 1 and 2 and aluminum in group 13 form
compounds in which the metal atom always has a positive
oxidation number equal to the number of its valence electrons
(ϩ1, ϩ2, and ϩ3, respectively).
8. The sum of the oxidation numbers in a neutral compound is
zero.
9. The sum of the oxidation numbers of the atoms in a polyatomic
ion is equal to the charge on the ion.
Many elements not specified in the rules above can have different
oxidation numbers in different compounds. The list of rules enables
you to determine these unknown oxidation numbers, as the following example problem demonstrates.
Example Problem 19-1
Determining Oxidation Numbers
Determine the oxidation number of each element in the following
compound and ion.
a. SrCO 3 (strontium carbonate)
b. Cr 2 O 7
2Ϫ (dichromate ion)
Assign the known oxidation numbers to their elements, set the sum
of all oxidation numbers to zero or to the ion charge, and solve for
the unknown oxidation number (n element ).
a. According to rule 8, the sum of the oxidation numbers is zero
because strontium carbonate is a neutral compound. Rule 5
states that the oxidation number of oxygen in compounds is
Ϫ2, and rule 7 states that group 2 metals, such as strontium,
have a ϩ2 oxidation number in compounds.