on the ion. In molecular compounds, oxidation numbers do not have the same signifi-
cance they have in binary ionic compounds. Oxidation numbers, however, are very useful
aids in writing formulas and in balancing equations. In molecular species, the oxidation
numbers are assigned according to an arbitrary set of rules. The element farther to the
right and higher up in the periodic table is assigned a negative oxidation number, and the
element farther to the left and lower down in the periodic table is assigned a positive
oxidation number.
Some rules for assigning oxidation numbers follow. These rules are not comprehen-
sive, but they cover most cases. In applying these rules, keep in mind two important points.
First, oxidation numbers are always assigned on a per atom basis; second, treat the rules
in order of decreasingimportance—the first rule that applies takes precedence over any
subsequent rules that seem to apply.1.The oxidation number of the atoms in any free, uncombined element is zero. This
includes polyatomic elements such as H 2 , O 2 , O 3 , P 4 , and S 8.2.The oxidation number of an element in a simple (monatomic) ion is equal to the
charge on the ion.3.The sumof the oxidation numbers of all atoms in a compound is zero.4.In a polyatomic ion, the sumof the oxidation numbers of the constituent atoms is
equal to the charge on the ion.5.Fluorine has an oxidation number of 1 in its compounds.6.Hydrogen has an oxidation number of 1 in compounds unless it is combined with
metals, in which case it has an oxidation number of 1. Examples of these excep-
tions are NaH and CaH 2.7.Oxygen usually has an oxidation number of 2 in its compounds. There are some
exceptions:
a.Oxygen has an oxidation number of 1 in hydrogen peroxide, H 2 O 2 , and in
peroxides, which contain the O 22 ion; examples are CaO 2 and Na 2 O 2.
b.Oxygen has an oxidation number of ^12 in superoxides, which contain the O 2
ion; examples are KO 2 and RbO 2.
c. When combined with fluorine in OF 2 , oxygen has an oxidation number of 2.8.The position of the element in the periodic table helps to assign its oxidation
number:
a.Group IA elements have oxidation numbers of 1 in all of their compounds.
b.Group IIA elements have oxidation numbers of 2 in all of their compounds.
c.Group IIIA elements have oxidation numbers of 3 in all of their compounds,
with a few rare exceptions.
d.Group VA elements have oxidation numbers of 3 in binarycompounds with
metals, with H, or with NH 4 . Exceptions are compounds with a Group VA
element combined with an element to its right in the periodic table; in this case,
their oxidation numbers can be found by using rules 3 and 4.
e.Group VIA elements below oxygen have oxidation numbers of 2 in binary
compounds with metals, with H, or with NH 4 . When these elements are
combined with oxygen or with a lighter halogen, their oxidation numbers can
be found by using rules 3 and 4.Polyatomic elements have two or more
atoms per molecule.
138 CHAPTER 4: Some Types of Chemical Reactions