Chapter 5 The Covalent Bond
OXIDATION STATE The
oxidation state
or
oxidation number
of atom A
(OX
) was defined in Section 4.4 as A
the charge the atom would have if all bonds were
ionic
; that is, if
all of the bonding
electrons were assigned to the more electronegative atom in each bond.
OX= VE - (NB + A∑ ajBE) jEq. 5.2The sum is over all bonds in which that atom is involved.
a = 1 if the atom is the more j
electronegative atom in the j
th bond, and
a = 0 if the atom is the less electronegative atom j
in the bond. In cases where the two bound atom
s are identical, the bonding electrons are
assigned to each atom equally (
i.e
.,
a = j½
).Using the method presented in Section 4.4 to
determine oxidation states leads to the
average oxidation state of each atom type in a compound, not to the oxidation state of each individual atom. This is the reason that fractional oxidation states are sometimes encountered. For example, the oxidation state of iron in Fe
O 3
as determined by the 4
methods of Chapter 4 is
8 /^3
, which is the average of the three iron atoms: two at +3 and
one at +2. Although, the method outlined in Chapter 4 is the more common, the method that uses Lewis structures is instructive and is presented here.
Consider the oxidation states of the atoms in ammonia. Nitrogen is more
electronegative than hydrogen, soa = 1 for N and
a = 0 for H and all six bonding electrons
are assigned to the nitrogen. The oxida
tion states are determined as follows:
OX= 5 - (2 + 6) = -3 OXN= 1 - 0 = +1 HNote that these values are the same as determined in Chapter 4. Example 5.8 a) Determine the oxidation state of carbon in acetic acid (C
2 H4 O2 ) using the rulesgiven in Section 4.4. The oxidation states of H and O are +1 and-2, respectively. The sum of the oxidationstates of the atoms in a molecule must sum to zero, so we write 2x + 4(1) + 2(-2) = 0, where x is the oxidation state of the carbon atom. Solving for x, we determine theoxidation state of carbon to be 0.
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