Lewis Structures
1
Lewis Structures
At the conclusion of our time
together, you should be able to:
1. List the basic rules for drawing a Lewis Dot
structure for a compound
2. Use these rules to draw a Lewis Dot
structure for a compound
2
Another New Element On The Periodic
Table
Element:
Man
Symbol:
Xy
Atomic Mass:
180 +/-100
Common Name(s):
3
Varies anywhere from
John to !@#$&*!
The Lewis Model of Chemical Bonding
In 1916 G. N. Lewis proposed that atoms
combine in order to achieve a more stable
electron configuration.
►
Maximum stability results when an atom
is isoelectronic with a noble gas.
►
An electron pair that is shared between
two atoms constitutes a covalent bond.
►
4
Covalent Bonding in H2
Two hydrogen atoms, each with 1 electron,
H.
H.
can share those electrons in a covalent bond.
H: H
► Sharing
the electron pair gives each hydrogen
an electron configuration analogous to helium.
5
Covalent Bonding in F2
Two fluorine atoms, each with 7 valence electrons,
..
..
: F. :
: F. :
can share those electrons in a covalent bond.
.. ..
: ..
F : ..
F:
► Sharing
the electron pair gives each fluorine
an electron configuration analogous to neon.
6
The Octet Rule
In forming compounds, atoms gain, lose, or
share electrons to give a stable electron
configuration characterized by 8 valence
electrons.
.. ..
: ..
F : ..
F:
7
Xy Continued:
Usage of Xy:
None really, except methane production. Good
samples are able to produce large quantities on
command.
Physical Properties:
Solid at room temperature, but easily gets bent out
of shape. Fairly dense and sometimes flaky. Difficult
to find a pure sample. Due to rust, aging samples
are unable to conduct electricity as easily as young,
fresh samples.
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Example
Combine carbon (4 valence electrons) and
four fluorines (7 valence electrons each)
..
.
. C ..
: F. :
to write a Lewis structure for CF4.
..
: ..
:
F
..
..
:
:
F
: ..F: C
.. ..
: F:
..
The octet rule is satisfied for carbon and
each fluorine.
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Example
It is common practice to represent a covalent
bond by a line. We can rewrite
..
.. : ..F: ..
:
: ..F: C
.. : ..F
: ..F:
..
: F:
as
..
: ..
F
C
: ..F:
10
..
..F:
Lewis Structures with
Double and Triple Bonds
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Inorganic examples
..
..
: O: : C : : O:
..
:O
C
..
O:
C
N:
Carbon dioxide
H : C : :: N:
H
Hydrogen cyanide
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Organic examples
H
.. H
..
H: C : : C:H
H
Ethylene
H
C
H
H
H : C : :: C:H
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Acetylene
H
C
C
C
H
Back to Xy
Chemical Properties of Xy:
Attempts to bond with Xx (Woman) any chance it can
get. Also, tends to form strong bonds with itself.
Becomes explosive when mixed with Kd (Kid) for a
prolonged period of time. Neutralized by saturating
with alcohol.
Caution:
In the absence of Xx (Woman), this element rapidly
decomposes and begins to smell.
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Rules for Lewis Structures
► 1.
Make certain that the bond is a covalent
bond then set up the skeleton structure as
follows:
 The atom with the lowest electronegativity
will tend to go in middle
 Place all the other atoms around this
central atom
 Attach these atoms to the central atom in
reasonable fashion with single bonds
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Rules for Lewis Structures
► 2.
Sum valence electrons
► 3. Complete octets of peripheral atoms
► 4. Place leftover e- on central atom
► 5. If necessary use multiple bonds to fill the
center atom's octet.
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For NF3
Like NBr3
Molecular
formula
Remaining
valence eLewis
structure
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:
: F:
:
: F:
N
N 5e-
F 7e- X 3 = 21e-
: F:
:
Sum of
valence e-
:
Atom
placement
Total
26e-
SAMPLE PROBLEM:
Writing Lewis Structures for
Molecules with One Central Atom
PROBLEM:
Write a Lewis structure for CCl2F2, one of
the compounds responsible for the
depletion of stratospheric ozone.
PLAN:
Follow the steps outlined previously
SOLUTION:
Step 1: Carbon has the lowest EN and is
the central atom. The other atoms are
placed around it.
Cl
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Cl
C F
F
SOLUTION:
Steps 2-4:
C has 4 valence e-, Cl and F each have
7. The sum is 4 + 4(7) = 32 valence e-.
:
Make bonds and fill in remaining valence
electrons placing 8e- around each atom.
:
C
:
: Cl
:
:F:
19
F:
:
:
: Cl :
SAMPLE PROBLEM: Writing Lewis Structures for
Molecules with Multiple
Bonds.
PROBLEM:
Write Lewis structures for the following:
Nitrogen (N2), the most abundant atmospheric
gas
PLAN:
If an atom does not have an octet, Step 5 which
follows the other steps in Lewis structure
construction must be done. If a central atom
does not have 8e-, an octet, then an e- can be
moved in to form a multiple bond.
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SOLUTION:
N2 has 2(5) = 10 valence e-.
N.
:
N.
:
:
: .
N.
N
N
:
.
:
21
N.
7th Grade Science Answers
"Blood flows down one leg and up the other."
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Lewis Structures
Let’s see if you can:
1. List the basic rules for drawing a Lewis Dot
structure for a compound
2. Use these rules to draw a Lewis Dot
structure for a compound
23
Rules for Lewis Structures
► 1.
Make certain that the bond is a covalent
bond then set up the skeleton structure as
follows:
 The atom with the lowest electronegativity
will tend to go in middle
 Place all the other atoms around this
central atom
 Attach these atoms to the central atom in
reasonable fashion with single bonds
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Rules for Lewis Structures
► 2.
Sum valence electrons
► 3. Complete octets of peripheral atoms
► 4. Place leftover e- on central atom
► 5. If necessary use multiple bonds to fill the
center atom's octet.
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How many lone pairs of electrons surround
the central atom in water H2O?
1.
2.
3.
4.
5.
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1
2
3
4
0
Try Some Problems on the Handout
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