Transcript Slide 1

Guidelines: Drawing Lewis Structures
1. Count total valence electrons in the molecule.
2. Arrange atoms into a "skeleton" molecule.
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The less electronegative element is usually the central atom.
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The element written first in the formula is usually the central atom.
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C bonds to other C atoms readily and forms 4 bonds.
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H forms 1 bond only
3. Connect all atoms with single bonds (dashes).
4. Distribute e-s to fill octets for all atoms (except H).
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recount your e-s
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If total e-s used (bonding and nonbonding) = valence e-s counted in step #1 you're done
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If you used more e-s to fill octets than were counted in step #1 create double or triple bond
5. Make sure to check your structure!!!
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Total e-s used in the Lewis Struct. should = total valence e-s counted in step #1
Examples: Drawing Lewis Structures
Molecular
formula
x
1 =
F
7e-
x
3 = 21e26e-
Total
: F:
: F:
:
Remaining
valence e-
5e-
:
Atom
placement
5e-
N
:
Sum of
valence e-
NF3
Total e-s needed = 26
: F:
:
Lewis
structure
N
Examples: Drawing Lewis Structures
PROBLEM:
Write a Lewis structure for CCl2F2, one of the compounds
responsible for the depletion of stratospheric ozone.
SOLUTION:
Step 1: C has 4 valence e-, Cl and F each have 7.
The sum is 4 + 4(7) = 32 valence e-
Steps 2-4: Carbon has the lowest EN and is the
central atom. The other atoms are placed around it.
Cl
Cl
C
F
F
: F:
:
C
F:
:
: Cl
:
around each atom.
:
-
: Cl :
:
electrons placing 8 e
:
Make bonds and fill in remaining valence
Examples: Drawing Lewis Structures
PROBLEM:
Write the Lewis structure for methanol (molecular
formula CH4O), an important industrial alcohol that is
being used as a gasoline alternative in car engines.
SOLUTION:
Hydrogen can have only one bond so C and O must be next
to each other with H filling in the bonds.
There are 4(1) + 4 + 6 = 14 valence eC has 4 bonds and O has 2. O has 2 pair of nonbonding e-
C
O
:
H
:
H
H
H
14 e-s total used to
fill all atom's octets
Examples: Drawing Lewis Structures (cont)
PROBLEM: Write Lewis structures for ethylene ( C2H4 ), the most
important reactant in the manufacture of polymers
PLAN:
C atoms easily bond to each other and H gets 1 bond only so
the C atoms are central and H are terminal
If you do not have enough e-s to fill all atoms octets
(exc H), create multiple bonds. Form a double bond if
short 2 e-s, and 2 double or a triple bond if short 4 es. Dbl bonds between C, N, S, O and triple bonds
between C, N, O
SOLUTION:
H
:
C
:
H
There are 2(4) + 4(1) = 12 valence eYou need 14 e- but only have 12. You’re short 2 e-s and
have to create a double bond
C
H
H
H
H
C
C
H
H
Examples: Drawing Lewis Structures
PROBLEM:
Write Lewis structures for the following:
Nitrogen ( N2 ), the most abundant atmospheric gas
SOLUTION:
-
:N
:
:
:
:
N2 has 2(5) = 10 valence e
14 e- are needed to fill octets
10 e- – 14 e- = 4 e-s short to fill octets
A triple bond is required to make the octet around each N
N:
:
N N:
Examples: Drawing Lewis Structures
Examples:
CCl4
SO2
NH3
SO3
H2O
CO2
PBr3
C2H5OH
H2CO3
H2CO
BeF2
BF3