Electron Bookkeeping: Lewis Dot and General Structure, Formal Charge and Resonance Contributors
Dr. Gergens - SD Mesa College
Determining formal charges on individual atoms in Lewis structures and drawing resonance contributors are
important elements in good "electron bookkeeping." So much of organic chemistry can be made more understandable by
keeping track of electrons that it is worth taking some time at the beginning to become proficient at the seemingly
simple task of counting electrons. This project is designed to review your current knowledge of electron counting used
in drawing Lewis dot chemical structure and to develop your skill in calculating formal charge about individual atoms
when drawing acceptable resonance structures.
Before discussing formal charge and resonance, let's review some fundamentals for drawing Lewis dot
structures. The covalent bond or shared electron pair model of chemical bonding was first suggested by G.N. Lewis of
the University of California in 1916. Lewis proposed that the sharing of electrons by two nonmetal atoms permits each
atom to have a stable closed-shell electron configuration. In forming compounds, atoms gain, lose, or share electrons
to achieve a stable electron configuration characterized by eight valence electrons as in a noble gas.
When the octet
rule is satisfied for carbon, nitrogen, oxygen and fluorine they have an electron configuration analogous to the noble
gas neon.
Representing a 2-electron covalent bond by a dash (—), the Lewis structures for the most stable bonding modes
for carbon, nitrogen, oxygen and hydrogen become:
••
:
H
C
H
C
C
H
C
C
H
O
H
methane gas
H
H
acetylene gas
ethylene gas
C
O
:
:
••
H
:
H
H
H
••
H C H
••
H
carbon dioxide gas
:
:
:
••
••
H
N
H
H
N
H
:
••
••
:
O
N
nitrogen gas
diazene gas
(unstable)
:
••
O H
••
N
H
H
ammonia gas
:
N
:
••
H N H
••
H
O
H
C
H
water liquid
:
H
H
formaldehyde gas
Multiple bonds are very common in organic chemistry, however, the most stable bonding mode for carbon is
one where carbon has four covalent bonded dashes (—) around it. Ethylene (C2 H4 ) contains a carbon-carbon double
bond in its most stable Lewis structure and each carbon has a completed octet. The most stable structure for acetylene
(C2 H4 ) contains a carbon-carbon triple bond and the octet rule is satisfied.
A systematic procedure for writing Lewis structures is outlined on the next page.
1
Summary of Steps for Drawing Lewis Dot Structures
Dr. Gergens - SD Mesa College
1.
Calculate the total number of valence electrons.
2.
Assemble the bonding framework.
3.
Connect the other atoms to the central by drawing a single line.
up of two electrons being shared between two atoms.
4.
Give the outer most atoms, EXCEPT for hydrogen, three sets of paired electrons.
provisional structure.
5.
Count valence electrons in your provisional structure.
are present.
6.
Add any missing electrons from your VE count in step 1 to the central atom.
7.
Recount the number VE in the structure and apply the octet rule to check to see that each atom has eight
electrons surrounding it.
8.
Share neighboring electrons by moving electrons to satisfy the octet about each atom.
9.
Place a bracket around ions, followed by the substances ion charge.
⊕
NH4
H
H
B
H
O
O
C
••
O
••
N
O
O
N
••
O
••
••
C
••
O
••
••
O
••
C
••
O
••
N
••
••
O
N
••
O
••
••
••
O
••
••
••
O
O
N
••
••
O
••
••
••
H
Rule 6
••
••
O
••
••
O
••
H
H
H
C
••
⊕
Rule 9
O
••
••
Rule 7
Rule 8
••
Rule 4
Rule 5
N
O
H
H
H
VE = 16
= 5
= 12
= 1
= 18
••
H
N
+
1N • 5
2O•6
+ 1 e–
VE
••
H
1C • 4 = 4
2 O • 6 = 12
H
H
H
3
4
1
8
H
B
H
N
••
O
••
••
C
1B • 3 =
4H•1 =
+ 1
=
VE =
••
H
5
4
-1
8
group
#
••
H
group
#
••
••
VE = 8
1N • 5 =
4H•1 =
+ 1 ⊕
=
VE =
NO2
••
••
= 4
= 4
CO2
group
#
group
#
+
Rule 2
Rule 3
BH4
••
1C • 4
4H•1
Rule 1
See if all valence electrons calculated in step 1
••
group
#
We now have a
••
CH4
Each line represents a single bond made
H
2
Summary of Steps for Calculating Formal Charge
Dr. Gergens - SD Mesa College
In this assigment you will get some more practice in drawing Lewis stuctures.
very important to the study of chemical structure.
This basic fundamental skill is
Certainly by the end of the this course you will be an expert at
drawing reasonable Lewis dot structures.
Computing the formal charge is one method to guide you in drawing reasonable Lewis structures and for
determining if the octet rule is applied. In general, the most reasonable Lewis structure is where (1) the octet rule is
obeyed over every atom, (2) the total formal charges are minimized, and (3) the more electron negative atom bears
the more negative formal charge.
Atoms with zero formal charge generally obey the octet rule and thus are in their
most stable form.
We calculate formal charges by counting the number of electrons "owned" by each atom in a Lewis structure
and comparing this electron count with that of a neutral atom.
The illustration below shows a method for calculating
formal charge.
A simple rule for calculating formal charges:
Formal Charge equals atomic group number minus dots and dashes
H
••
C
N
N
••
FC = atomic group number - touch, where the touch equals dots
and dashes
Formal Charge Example
Diazomethane, CH2N2
FC = Grp# - (dot & dashes)
= 5
- ( 0 + 4 )
FC = 1⊕
H
FC = Grp# - (dot & dashes)
= 5
- ( 2 + 3 )
FC = 0
FC = Grp# - (dot & dashes)
= 4
- ( 2 + 3 )
FC = 1
The illustration can be summarized as the "group number minus the touch" rule, FC = Grp # - TC. The number of
touches is calculated by counting all electron dots and dashes directly around a single atom.
By subtracting the total
sum of dots and dashes from the representative group number (groups IA - VIIIA) for the atom in a Lewis structure, a
formal charge for that atom is calculated.
charge of the substance formula.
Finally, the sum of the formal charges on all atoms must add up to the
As seen in the example above, the formula for diazomethane, CH2 N2 , is a neutral
compound, but in its structure the carbon bears a negative and nitrogen a positive formal charge.
charges is equal to the overall charge of the substance.
The sum of the two
Later we will see that the Lewis structure now drawn for
diazomethane, although a reasonable structure, is not the preferred structure, but that electrons may relocate through
resonance electron delocalization to produce a even more stable Lewis structure.
By now you are very aware of the preferred bonding modes for carbon, nitrogen, oxygen, and hydrogen.
On
the next page are additional bonding modes for these atoms when they are formally charged. Please go through each
structure and verify that the formal charges were calculated correctly by using the "group number minus the touch"
rule, FC = Grp # - TC.
3
Summary of Bonding Modes and Formal Charges
Dr.Gergens - SD Mesa College
Formal Charge of zero, 0
a generally perferred molecular and atom charge
H
H
H
Formal Charge of 1+
:
H
:
H
C
C
H
C
C
H
O
H
H
C
O
H
H
:
C
H
H
H
H
H
:
⊕
N
N
H
H
H
⊕
O
H
H
A simple rule for calculating formal charges:
N
H
⊕
O
⊕
N
H
C
H
:
O
H
N
H
⊕
H
:
H
N
H
:
O
N
:
:
N
H
C
:
N
⊕C
carbon
disobeys
octet rule
H
:
H
H
:
C
H
H
:
N
H
H
H
:
H
⊕
C
:
:
C
⊕
C
:
H
not preferred for carbon
Formal Charge of 1-
not preferred for carbon
:
H
:
Formal Charge equals atomic group number minus dots and dashes
H
C
H
C
:
C
FC = atomic group number - (dots and dashes)
H
H
H
C
C
H
carbon
doesn't
like to be
negative
Formal Charge Example
N
H
H
N
:
N
:
FC = Grp# - (dot & dashes)
= 5
- ( 2 + 3 )
FC = 0
N
H
:
FC = Grp# - (dot & dashes)
= 4
- ( 2 + 3 )
FC = 1
:
O
H
:
H
N
••
C
:
••
:
H
FC = Grp# - (dot & dashes)
= 5
- ( 0 + 4 )
FC = 1⊕
:
Diazomethane, CH2N2
4
Geometry and Molecular Polarity - Concept Map
Dr. Gergens - SD Mesa College
Lewis Dot
molecular
compounds
Molecular
Geometry
Non-Ideal Geometry
lone pair of electrons
on the central atom
tetrahedral
trigonal
planar
X
X
linear
linear
..
X
pyramidal
Molecular Polarity
for Ideal Geometries
X
bent
X
..
..
..
X
X
..
X
..
X
..
Ideal Geometry
no lone pair of electrons
on the central atom
bent
Molecular Polarity
for Non-Ideal Geometries
symmetrical
nonpolar
asymmetrical
polar
H
H X H
H
F
H X F
H
tetrahedral
tetrahedral
in general, non-ideal geometries
give rise to polar molecules
DG © 2002
5
VSEPR = _____________
_____________
_____________
_____________
_____________
spell out the meaning of vsepr
Ideal Geometries (no electron pair on centeral atom)
H
H
H
:
C
H
C
C
C
C H
:
N
N
O
C
O
:
H
::
H
H
H
H
give geometric name about each atom
give bond angles about each atom
Non-Ideal Geometries (electron pair on centeral atom)
:
:
:
N
H
H
N
N
:
H
H
H
give geometric name about each atom
give bond angles about each atom
:
:
H
O
H
H
O
:
:
C
H
give geometric name about each atom
give bond angles about each atom
6
DIPOLE MOMENTS & MOLECULAR POLARITY
Determine the bond polarity between two disimilar atoms in a chemical bond. Draw in all bond dipole moments and the overall dipole moment if the molecule is polar.
H
C
H
Cl
Cl
H
C
H
H
H
H
Cl
Cl
C
H
C H
H
C
H
C
H
C
H
O
H
O
Cl
C
Cl
C
C
Cl
trans
cis
:
:
:
H
N
H
H
N
H
:
N
N
N
:
H
cis
:
H
N
N
:
H
trans
:
H
:
:
O
H
H
O
:
C
C
C
H
H
H
H
H
C
H
7
Only this portion of the laboratory will be graded. You may want to draw your structures on
scratch paper first, then neatly transfer your answers to these pages
The Assignment - answer the questions YOUR NAME _______________________________
1) Write the most stable Lewis structure for each of the following:
CCl4
VE = ___
HOCl
VE = ___ SO2
carbon tetrachloride
hypochlorous acid
+
VE = ___
NO
N3
4-
VE = ___ C
azide ion
VE = ___
PH3
NH4
VE = ___ NH2
carbide ion
ozone
sulfur dioxide
nitrosonium ion
–
VE = ___
VE = ___ O3
smog
"bleach"
+
VE = ___
+
H3O
ammonium ion
–
hydronium ion
–
VE = ___ HO
amide ion
VE = ___
phosphine
NO2
+
VE = ___
nitronium ion
VE = ___
hydroxide ion
state the geometry about the central atom in
a.carbon tetrachloride
b. bleach
c. ozone
d. phosphine
e. ammonium ion
f. hydronium ion
g. nitronium ion
h. azide ion
i. amide ion
8
2) Write the most stable Lewis structure for each of the following polyatomic ions:
BO3
3-
VE =___
borate ion
CO3
2-
VE =___
NO3
carbonate ion
-
VE =___
nitrate ion
PO4
3-
VE =___
phosphate ion
SO4
2-
VE =___ ClO4-
sulfate ion
3) Write the most stable Lewis structure for each of the following oxyacids.
VE =___
perchlorate ion
In general, acids that contain oxygen
atoms in their formula will have their hydrogens attached to the oxygen atoms:
H2CO3
VE =___ H3PO4
carbonic acid
carbonated water
VE =___ H2SO4
phoshoric acid
tartness in Coke
VE =___
sulfuric acid
car battery acid
4) Write the most stable Lewis structure for each of the following (Hint check your work @ www.chemfinder.com):
a.
Acetaldehyde, CH3 CHO. Three hydrogens are bonded d.
to one carbon. (Similar to formaldehyde but contains
CH3 — methyl branch.)
Acylonitrile,
C3 H3 N, a starting material for the
preparation of acrylic fibers such as Orlon and
Acrilian. The atoms are connected in the order CCCN.
b.
Tetrafluoroethylene, C 2 F4 . The starting material for e.
the preparation of Teflon.
MTBE, methyl t-butyl ether, (CH3 )3 COCH3 , a fuel
additive. There are four CH3 — methyl branches.
c.
The air we breath consists of 78% nitrogen and
21% oxygen. Draw the Lewis structures of N 2 & O 2.
CO2 and CH4 are molecules that
"Greenhouse" effect of atmosphere.
structures.
f.
add to the
Draw their
9
Formal Charge, Geometry, & Chemical Formulas
4) Assign the proper formal charge to the carbon, nitrogen, and oxygen atoms in each of the following structures:
"group number minus the touch" rule, FC = Grp # - TC.
5) Assign geometric name to each atom where an arrow is pointing
••
••
O
••
••
••
O
••
O
N
⊕
O
H
N
⊕
H
••
••
C
••
••
••
••
••
••
••
••
••
O
O
C
O
••
H
C
H
H
C
••
H
H
A
H
H
C
C
H
••
C H
N
H
••
H
H
N
⊕
N
••
O
⊕ H
H
H
H
••
C
••
••
N
N
H
H
H
B
••
••
••
••
••
O
O
H
••
••
••
••
••
••
••
O
P
O
••
O
O
H
O•
•
••
••
••
6) Write the chemical formula and overall charge for the substances label A, B, C above.
hydrogen attach to oxygen are acids and have hydrogen listed first in theirchemical formula.
C
H
••
••
••
O
••
O
••
••
••
••
••
••
H
••
S
••
H
O
••
••
O
••
••
••
2+
H
⊕
N
N
H
O
C
O
••
H
S
⊕
••
••
••
••
O
O
H
••
O
••
••
O
H
O
••
C
••
C
Substances that have
Substances that have
carbon, carbon is listed first in the formula, hydrogen next, followed by nitrogen then oxygen.
A.
B.
C.
10
7) Two structures can be written for the molecular formula
C4H10 .
This is called constitutional isomerism. Note
every carbon receives for bonds and each carbon in this structure has a tetrahedral geometry.
H
H
H
C
H
H
H
H
Draw all structural isomers for
H
H
C
H
H
H
C3H6.
H
C
C
C
H
a.
H
C
C
C
H
H
H
H
H
(Hint: there are three constitutional isomers, one of which has is a
three carbon ring, and all have multiple bonds.)
b.
Give the geometry about each atom in all three isomers. Use an arrow,
, to point to each carbon atom
when identify the geometry.
11