Formal charge
Draw a Lewis dot structure for ClO-.
Which atom has the charge?
 A formal charge is a positive or negative charge on an individual
atom
 The formal charge is calculated by subtracting the assigned
electrons on the atom in the molecule from the electrons in the
neutral atom
 Electrons in bonds are evenly split between the two atoms; one to
each atom
 Lone pair electrons belong to the atom itself
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Formal charge
The sum of formal charges on individual atoms is the total
charge of the molecule or ion
 Examples
◦ Ammonium ion (NH4)+
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EXAMPLE 10-7
Using the Formal Charge in Writing Lewis Structures. Write
the most plausible Lewis structure of nitrosyl chloride, NOCl,
one of the oxidizing agents present in aqua regia.
2+
2
2-
-
2+
-
-
+
As a general rule, we want to keep the formal charge on each atom as
close to 0 as possible (without giving any atom more than a complete
octet).
Which Lewis Structure Is Valid?
Draw a Lewis dot structure for NO2+.
 Sum of FC is the overall charge.
 FC should be as small as possible.
 Negative FC usually on most electronegative elements.
 FC of same sign on adjacent atoms is unlikely.
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Formal charge calculation helps to determine location of multiple bonds
Formal charge pairs can be replaced by forming multiple bonds.
This helps to determine the location of the multiple bond.
 Nitrous Acid HNO2
 Nitrate ion (NO3)-
-1
O
-1
O
5
N
+1
0
O
A Summary of Formal Charges
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An atom will always have the same formal
charge depending on how many bonds and lone
pairs it has regardless of which particular molecule it is in
Resonance
How many Lewis structures can you draw for ozone?
What is the average bond order for an O-O bond in ozone?
-½ ••
••+ •• -½
O
O O
••
••
If more than one valid Lewis structure can be drawn for a molecule, the true
electron distribution is a hybrid of the possible distributions. This is called
resonance.
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Resonance Structures
 Draw the best Lewis dot structure for nitrate (NO3-).
Note that there are actually ____ equally correct answers.
 When you can draw different valid Lewis dot structures by
moving electrons but not atoms, they are called resonance
structures.
 What is the average bond order for a N-O bond in nitrate?
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Exceptions to the Octet Rule
 Incomplete octets.
••
••
••
F
+ F
••
••
••
B
B
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F
F
F
F
••
••
••
+
-
F
F
-
B
F
Exceptions to the Octet Rule
 Expanded octets.
P
Cl
F
S
P
Cl
Cl
Cl
F
F
F
••
Cl
••
••
Cl
F
••
Cl
••
••
Cl
••
••
••
••
••
••
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F
Expanded Valence Shell
How many resonance
structures for SO42-?
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Polyatomic Ions
What are they???
A group of atoms with a charge.
3 special ions:
ammonium
hydroxide
cyanide
5 -ate:
nitrate
chlorate
carbonate
sulfate
phosphate
4 -ite:
nitrite
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chlorite
sulfite
phosphite
Polyatomic Ions
Adding one “H” takes away one “-” charge.
5 -ate:
NO3-
ClO3-
4 -ite:
NO2-
ClO2-
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CO32-
SO42-
PO43-
SO32-
PO33-
Practice
Write formulas including the charge for the following
polyatomic ions:
bicarbonate
ammonium
hydrogen sulfate
phosphate
hydroxide
sulfite
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In-Class Practice
Write the Lewis structure for the polyatomic ion in each
and name the compound:
Na2SO4
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In-Class Practice
Write the Lewis structure for the polyatomic ion in each
and name the compound:
Li3PO4
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In-Class Practice
Write the Lewis structure for the polyatomic ion in each
and name the compound:
NaHCO3
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Recap
•
Formal charge
•
Resonance
•
Expanded valence shell
•
Polyatomic ions
Lewis dot structures do not show a
molecule’s shape.
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Readings for Next Class
10.7
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