9/30/2014
ALEKS Alex H. Yuan - 09/30/2014 10:58:05 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
Alex H. Yuan - 09/30/2014 10:58:05 PM EDT
General Chemistry (First Semester) / Chem 110 FA 2014 – CH110 FA14 S25-36 (Dr.
Barber)
1.
Calculating formal charge
A student proposes the following Lewis structure for the isocyanate
N
C
ion.
O
Assign a formal charge to each atom in the student's Lewis structure.
atom formal charge
N
C
O
Chemists assign formal charges to get a rough idea of how much the arrangement of
valence electrons near an atom changes when the atom forms a chemical bond. Generally
speaking, the larger the formal charge the more the arrangement changes. Since changing
the arrangement of valence electrons a lot costs more energy, chemical bonds that result in
large formal charges are less likely to form naturally.
You can use this fact when you have to choose one Lewis structure from among several
different possibilities: The Lewis structure with the smallest formal charges will usually be
the best. If there are several possible structures with the same size of formal charges, it's
best to pick the structure in which negative formal charges are assigned to the most
electronegative atoms.
You can assign a formal charge to an atom by subtracting the number of valence electrons
(VE) assigned to the atom in the molecule from the number of valence electrons in the free
atom:
VE in free atom
VE assigned to atom in molecule
https://secure.aleks.com/alekscgi/x/Isl.exe/1o_u-IgNsIkr7j8P3jH-lJiYsPgPQIfQokR4hWMNIe-wKsFh3XosS7WHcmXKZd4GiLU3qxlZmco8pYUac1kKlp…
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9/30/2014
ALEKS Alex H. Yuan - 09/30/2014 10:58:05 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
formal charge
To assign valence electrons to an atom in a molecule, use these rules:
Assign all electrons of a lone pair to the atom it's on.
Assign half the electrons of a bonding pair to the atom it's on.
Now use these general ideas to assign formal charges to the atoms in the student's
proposed Lewis structure for the
atom
N
ion:
Lewis structure
(assigned electrons
highlighted)
N C
O
formal charge calculation
VE in free N atom
VE assigned to N in
formal charge
C
N C
O
VE in free C atom
VE assigned to C in
formal charge
O
N C
O
VE in free O atom
VE assigned to O in
formal charge
Note: An important check on your assignment of formal charges is this: the sum of all
formal charges in a molecule must equal the total charge on the molecule. Because the
charge on the
add up to
ion in this problem is
, the formal charges in this problem must
Here's the completed table:
atom formal charge
N
C
O
https://secure.aleks.com/alekscgi/x/Isl.exe/1o_u-IgNsIkr7j8P3jH-lJiYsPgPQIfQokR4hWMNIe-wKsFh3XosS7WHcmXKZd4GiLU3qxlZmco8pYUac1kKlp…
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9/30/2014
ALEKS Alex H. Yuan - 09/30/2014 10:58:05 PM EDT - Copyright © 2014 UC Regents and ALEKS Corporation
Copyright © 2014 UC Regents and ALEKS Corporation
https://secure.aleks.com/alekscgi/x/Isl.exe/1o_u-IgNsIkr7j8P3jH-lJiYsPgPQIfQokR4hWMNIe-wKsFh3XosS7WHcmXKZd4GiLU3qxlZmco8pYUac1kKlp…
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