Chemistry Reference
In-Depth Information
contain covalent bonds), however, we need oxidation numbers. Oxidation num-
bers for such species must be assigned by allocating the shared electrons in the
covalent bonds to one or the other of the atoms, or equally to both if the atoms
are the same. If the atoms are identical, they each control half of the shared elec-
trons. If they are different, we arbitrarily allocate all the electrons in the bond to
the more electronegative of the atoms involved (see Figure 5.4). In most cases,
that will be the atom farther to the right or farther toward the top of the periodic
table. The electrons are assigned to the more nonmetallic of the elements involved
in the bond. For example, in the bonds are covalent, so there are no charges.
Because nitrogen lies closer to the metal area of the periodic table, each fluorine
atom is assigned
“control” of the shared electrons.
This control is not complete;
if the fluorine atoms really had complete control of the electrons, they would be
fluoride ions. This compound is covalent. However, the concept of control is use-
ful for the purposes of assigning oxidation numbers.
We can assign oxidation numbers from the electron dot diagram of a com-
pound, such as
“Control” of the electrons in a
covalent bond is arbitrarily
assigned to the more electro-
negative element of the bond.
NF
3
NF
3
:
F
F
N
F
The nitrogen atom, when uncombined, has 5 valence electrons. In the com-
pound, the shared electrons are “controlled” by the fluorine atoms, so the nitro-
gen atom retains “control” of only the unshared pair of electrons. It “controls”
only 2 electrons. Its oxidation state is The uncombined fluorine
atom has 7 valence electrons; but in the compound, each fluorine atom “con-
trols” 8, so its oxidation number is
5
2
3.
7
8
1.
Nitrogen
Fluorine
N
free
5
7
minus
N
controlled
2
8
Oxidation number
3
1
Note that an oxidation number, like the charge on an ion, is assigned to
each atom
in a compound. Thus,
each
fluorine atom in has an oxidation number of
In most cases, assigning oxidation numbers from formulas by drawing elec-
tron dot diagrams is more time-consuming than necessary. The following set of
rules speeds the assignment of oxidation numbers for the elements in most
cases. If these rules do not work for assigning oxidation numbers in a particu-
lar case, the electron dot method may be used.
NF
3
1.
Rules for Assigning Oxidation Numbers
1. The total of the oxidation numbers of all the atoms (not simply all the ele-
ments) is equal to the charge on the atom, molecule, or ion. Thus, for any
compound, the sum of the oxidation numbers totals zero because the net
charge on any compound is zero. The sum of the oxidation numbers of the
atoms of any polyatomic ion is equal to the charge on that ion.
2. The oxidation number of any uncombined element is zero. Because the ele-
ment is not combined with any other element, there is either one atom alone
or a group of identical atoms. Because the atoms are alike, the electrons are
shared equally, and the number “controlled” is equal to the number in the free
