THE CONCEPT OF RESONANCE (9.8)
STUDY OBJECTIVES
- Draw resonance structures for molecules or polyatomic ions.
- Determine the formal charges on atoms in a resonance structure.
Resonance Structures.
It sometimes happens that a satisfactory electron dot structure for a molecule
or polyatomic ion cannot be drawn. When such a situation arises a special
procedure is invoked to arrive at a Lewis structure. For the nitrite ion ,
for instance, the following structure shows the correct number of valence
electrons and satisfies the octet rule. The brackets are used to indicate
that the –1 charge belongs to the entire nitrite ion, and not to just one
atom in the structure.
However, the structure does not accurately represent what is known about
the bond lengths of the N—O bonds in . Both bond lengths are known to be the
same, whereas according to the structure we expect the double bond to be shorter
than the single bond. An N—O single bond length should be about 136 pm, and
an N
O
double bond length about 122 pm. However, the two bond lengths in are equal
and are intermediate between these two values.
It turns out to be impossible to draw a single satisfactory Lewis structure
for . Situations like this can be handled by using the concept of resonance.
First draw two structures for that reflect different choices of electron arrangements.
The nitrite ion is not adequately represented by either structure; but it
may be described by a composite of these structures. This composite structure
cannot be drawn by the rules for writing Lewis structures. Each of the structures
that contributes to the composite structure is called a contributing
or resonance structure. The
symbol‚
indicates that the structures shown are resonance structures.
In applying the concept of resonance we assume that is a composite or an
average of the two structures. Thus, the N—O bonds are intermediate between
single and double bonds. The term resonance was perhaps a poor choice because
the word implies to some that the real molecule flips from one structure to
the other. This is not what is implied here. The point is that the
properties of cannot be accounted for by a single Lewis structure. The term
resonance means the that there is a need for two or more Lewis stuctures
to represent a particular molecule.
These structures have the same placement of atoms but different positions
of electron pairs. The electron pairs do not actually move from one position
to another within the molecule. Therefore resonance structures are generated
by moving electron pairs within the same skeletal structure.
EXAMPLE Drawing a Lewis Structure with Resonance
Draw three resonance structures for N2O. The skeletal structure
is N—N—O.
Correct!
Click
a Hint button for help.
-
Arrange the atoms
N—N—O
-
Count the valence electrons.
2(5) + 6 = 16 electrons
-
Add unshared pairs to the terminal atoms:
- Count the electron pairs used. 8 electron pairs = 16 electrons. There
are not enough electrons to add any to the central atom.
-
Move one of the unshared pairs from the terminal N atom to make a double
bond, and move an unshared pair from the terminal O atom to make a double
bond.
This is a satisfactory structure because it completes the octets of all
three atoms and uses the correct number of electrons. Additional resonance
structures can be generated by moving electron pairs in such a way that the
octet rule for each atom is always satisfied. The positions of the atoms in
N2O cannot be altered.
To possibilites are
•Comment
The three structures for N2O are called resonance structures.
The actual bonding in N2O is a composite of these three structures.
OBJECTIVE CHECK
Complete the following questions to check your understanding
of the material. Select the check button to see if you answered correctly.
- Draw resonance structures for nitric acid:
