Linus Pauling: A very important use of valence, old fashioned valence, was in balancing equations
for oxidation-reduction reactions. Now that valence, the old idea of valence, has
been replaced by a number of new and more precise concepts, we have still a problem
of doing something about oxidation-reduction reactions, and this problem has been
solved by introducing another concept, the concept of oxidation number.
Oxidation numbers assigned to atoms permit us to keep track of the electrons in an
easy way. Whereas the other aspects of valence that we have been discussing are less,
are less artificial, are really an effort to understand what nature is like, just
what the electrons are doing in a complex such as the cobaltic hexammine ion, oxidation
number is, by nature, artificial. It is something that we introduce with the use
of certain rules.
One of the rules about oxidation number is that in a, in an element, the oxidation
numbers of the atoms are zero. For example, in the chlorine molecule, Cl2, each chlorine atom started out with seventeen electrons, two of the electrons are
shared, but we split those two electrons between the two atoms so that each atom still
has seventeen, no resultant charge, and we say that for Cl2 in Cl, the oxidation number of each atom is zero. Similarly for hydrogen, similarly
for copper, or aluminum, or any other metal, the electrons are divided up equally
among the atoms, and in as much as each atom is, in as much as the whole metal is
electrically neutral, the individual atoms must be electrically neutral too.
In the case of a molecule such as H2O, O, H, H, the electrons that are shared between two atoms of different elements,
oxygen and hydrogen, are assigned to the more electronegative of the elements. Now,
oxygen, you remember the electronegativity scale, oxygen is more electronegative than
hydrogen. We assign these electrons to oxygen so that oxygen becomes O-2, hydrogen
is H plus one. The sum of the oxidation numbers in the molecule must add up to zero
because the molecule is electrically neutral. If we consider the hydronium ion, H,
H, H, here, the hydronium ion that has a plus charge, again we assign the electron
pairs that are shared with hydrogen to the oxygen atom. Oxygen has oxidation number
minus two, hydrogen plus one, hydrogen plus one, hydrogen plus one, and, the sum of
the oxidation numbers for all the atoms adds up to plus one, which is the charge on
the complex ion.
With, now, hydrogen peroxide, H2O2, the oxidation numbers can be assigned in either one of two ways. We may say that
hydrogen has oxidation number plus one, the two oxygen atoms are equivalent, therefore
in order that the molecule be electrically neutral, oxygen must have oxidation number
minus one. Or, knowing the structure of the molecule, it has this structure, in fact,
it is known that there is a dihedral angle between the H-O-O plane and the other H-O-O
plane about us shown here. O-O, distance, well, I don’t, 1.47 angstrom, O-H distance,
.96 angstrom, and so on, and the electronic structure that we can assign to it is
this one. This pair of electrons is to be assigned to this oxygen atom. The shared
pair split so that this oxygen atom has a total of seven, one, two, three, four, five,
six, seven, yes, seven electrons surrounding the nucleus, and of course, the two electrons
of the helium shell also. This means that the oxygen atom ends up with a charge of
minus one. The oxidation number of oxygen is minus two in almost all oxides. Minus
one in the peroxides were there is a O-O single bond, and zero in the elementary substance.
We can discuss oxidation numbers of other elements without discussing, in certain
compounds, without discussing the distribution of the electrons. For example, consider
the permanganate ion, MnO4 with a charge of minus one. Oxygen, this is not a peroxide,
the properties are not those of a substance containing an oxygen-oxygen single bond,
and so we assign to oxygen the oxidation number minus two. There are four of these
oxygen atoms. This gives a total eight negative charges; one of them still remains
in the ion itself. Hence, manganese must be plus seven.
Well, of course, with the electronic structure, whatever the electronic structure
is, each oxygen atom must have four electron pairs in its valence shell, either shared
or unshared, some of them probably shared with manganese, but oxygen is much more
electronegative than manganese, manganese is somewhere in this region, so that we
would assign these electron pairs to oxygen and not to manganese. All seven valence
electrons of manganese have been taken away from it, it is manganese with oxidation
number plus seven.