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that valence, the old idea of valence, has been replaced by a number of new and more precise concepts, we have still a problem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
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.