Page 120 - Basic physical chemistry for the atmospheric sciences

1 0 6 Basic physical chemistry

from + 4 to + 6 or, stated another way , from changing its oxidation
state from S(IV) to S(VI) - read "sulfur four to sulfur six." 2
The (arbitrary) rules used in assigning oxidation numbers are as
w
follo s :
o
1 . The oxidation number f a monoatomic substance is the charge on
2
the atom [e.g. , Cu + (aq) and s - ( aq) have oxidation numbers of + 1
and - 2, respectively] .
s
2 . In ionic binary compound , the oxidation numbers are the charges
per ion. [For example, CdCl2 i s an ionic compound, as indicated
2
more clearly by Cd + ( cl - ) z . Thus, the oxidation number of the
cadmium ion is + 2, and the oxidation number of each of the two
chloride ions is - l .] The algebraic sum of the oxidation numbers
of the atoms in an ion is equal to the charge on the ion (e.g. , zero
charge for CdCl2) .
3 . In nonionic (covalent) compounds , the electrons involved i n bond
formation are shared , more or less equally, by the bonding atoms.
However, to assign oxidation numbers, it is assumed that each
bonding electron is attached to a particular atom. If these atoms are
identical, the bonding electrons are shared equally between the two
atom . If the atoms are different, all of the electrons in the bond
s
are assigned to the atom that has the greater "attraction" for elec­
trons (as indicated by its electronegativity). The most electronega­
tive elements , in order of decreasing electronegativity, are F, 0, N ,
C
and l . Nonmetals are more electronegative than metals. A partial
list of electronegativities is given in Table 6. 1 .
These definitions lead to the following rules for assigning oxidation
numbers in polyatomic molecules .

1 . The oxidation number of all elements in their elementary state or in
any self-binding form is zero (e.g. , H i n H , 0 in 02 , S i n S o r S8).
2
2 . The oxidation number of the oxygen atom is - 2 i n all of its com­
pounds (except peroxides, such as H202 , where it is - 1 , and in the
elementary and self-binding forms of oxygen, where it is zero).
3 . The oxidation number of hydrogen is + 1 in all of its compounds
(except those with metals , where it is - 1 , and in the elementary and
h
self-binding forms of y drogen, where it is zero).
4 . All other oxidation numbers are assigned in such a way as to make
the algebraic sum of the oxidation numbers equal to the net charge
n
on the molecule or io .