Formal Charge And Resonance - Chemistry Worksheet Page 3
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4Chemistry Resource
NAME: _____________________________
Block: _________
Formal Charge and Resonance
–
Example: Draw the resonance structures of nitrate (NO
).
3
–1
. . –1
. . –1
:O:
:O:
:O:
||
|
|
↔
↔
N
N
N
/
\
//
\
/
\\
:O:
:O:
:O:
:O:
:O:
:O:
' '
' '
' '
' '
The three resonance structures are shown in the figure. Note that only the locations of double and
single bonds change here. What are the formal charges for the N atoms? What are the formal
charges for the oxygen atoms that are single bonded and double bonded to N respectively?
Please work these numbers out.
o Formal charges: N, +1; =O, 0; –O, –1
o The most stable structure has the lowest formal charge values (minimum separation
of charge).
o In a stable structure, adjacent atoms should have formal charges of opposite signs.
The more stable the structure, the more it contributes to the resonance structure of the
molecule or ion. All three structures above are the same, only the double bond rotates.
Exceptions to the octet rule
We can write Lewis dot structures that satisfy the octet rule for many molecules consisting of
main–group elements, but the octet rule may not be satisfied for a number of compounds. For
example, the dot structures for NO, NO
, BF
(AlCl
), and BeCl
do not satisfy the octet rule.
2
3
3
2
..
.
N:::O:
.
:F:
N
|
. .
. .
compare
//
\
:Cl : Be : Cl:
B
with
:O:
:O:
/
\
' '
' '
' '
:F:
:F:
:C:::O:
' '
' '
The above are structures for the gas molecules. The solids of AlCl
and BeCL
are polymeric
3
2
with bridged chlorides.
. .
. .
:Cl: :Cl: :Cl:
:Cl: :Cl: :Cl:
Polymeric
\ / \ /
/
\ / \ /
\
solid
Al
Al
Be
Be
structures
/ \ / \
\
/ \ / \
/
:Cl: :Cl: :Cl:
:Cl: :Cl: :Cl:
. .
. .
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