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Formal ChargeEdward A. Mottel
Department of Chemistry
Rose-Hulman Institute of Technology
04/18/23
Formal Charge
The formal charge is the net charge on each atom.
Assume bonding electronsbelong 1/2 to each bonding atom.
HC
H
H O
H
• •• •
04/18/23
Formal Charge
formalcharge
unsharedelectrons= e– donated - e– shared
2-
formal chargeof oxygen
6 valenceelectrons
=4/2 sharedelectrons
- -4 unsharedelectrons
HC
H
H O
H
• •• •
04/18/23
• •• •
••C OHH
HH••••
• •• •
4 - 8/2 - 0 = 0
••••
• •• •
••••
• •• •
••
• •• •
••1 - 2/2 - 0 = 0
••
6 - 4/2 - 4 = 0
• •• •
•• ••
Formal Charges of AtomsMethanol
Formal charge = valence electrons - electrons shared/2 - electrons not shared
C: 4 - 8/2 - 0 = 0
H: 1 - 2/2 - 0 = 0
O: 6 - 4/2 - 4 = 0
Atoms with low formal charges generally indicate thecorrect bonding pattern of chemically stable molecules.
04/18/23
Incorrect Diagrams for Methanol
••O• •
• •••••H
H• •
H
• •C HC: 4 - 6/2 - 2 = -1
H: 1 - 2/2 - 0 = 0
O: 6 - 6/2 - 2 = +1Validity Check
The sum of the formal charges always equalsthe charge on the molecule or ion.
HCH O
H
H
• •• •
C: 4 - 4/2 - 4 = -2
H: 1 - 2/2 - 0 = 0
O: 6 - 8/2 - 0 = +2
What are the formal charges for each atomin the following structures?
Validity CheckThe sum of the formal charges always equals
the charge on the molecule or ion.
04/18/23
Incorrect Diagrams for Methanol
••O• •
• •••••H
H• •
H
• •C H HCH O
H
H
• •• •
C: 4 - 4/2 - 4 = -2
H: 1 - 2/2 - 0 = 0
O: 6 - 8/2 - 0 = +2
C: 4 - 6/2 - 2 = -1
H: 1 - 2/2 - 0 = 0
O: 6 - 6/2 - 2 = +1
••O• •
• •••••H
H• •
H
• •C H
If either of these structures was stable,which one would more likely be stable?
04/18/23
C N
Cyanide Ion
N 5 valence e–
total 10 valence e–
C 4 valence e–
5 e– pairs
Skeleton diagram
What are the formal charges for each atom in this ion?
C N•• ••••
charge 1 e–
•••• •• •••••• •• •••••• •••••• ••
Convert to aLewis Dot diagram
C: 4 - 6/2 - 2 = -1N: 5 - 6/2 - 2 = 0total charge = -1
Lewis Dot diagram
04/18/23
Ammonium Ion
Nitrogen usuallyforms 3 bonds
NH H
H
H +
• • N 5 valence e–
total 8 valence e–
H 4 valence e–
charge -1 e–
Skeleton diagramLewis Dot diagram
4 e– pairs
••N• •
• •••H H
H
H +
H: 1 - 2/2 - 0 = 0
N: 5 - 8/2 - 0 = +1
total charge = +1
Which atoms have a non-zero formal charge?In an ion, some atom must have a non-zero formal charge.Why?
04/18/23
Formal Charges
What are the formal charges on the atoms in• hydroxide ion• hydronium ion (oxonium ion)
04/18/23
Chlorate Ion
Cl-O-O-O –
–
OO
O
Cl
Skeleton diagram
two possibilitiesusually the unique atom is in the center
chain structuresare uncommon
correctstructure
04/18/23
Chlorate Ion
O 18 valence e–
total 26 valence e–
charge 1 e–
Cl 7 valence e–
Only 6 electronsshown
(3 pairs)
OO
O
Cl
Skeleton diagramLewis Dot diagram
13 e– pairs
••O
• •• •
••••O• •
• •••
••O
• •• •
••
• •Cl
Cl: 7 - 6/2 - 2 = +2
O: 6 - 2/2 - 6 = -1
total charge = -1
O: 6 - 2/2 - 6 = -1O: 6 - 2/2 - 6 = -1
Why is chlorate ion a good oxidizing agent?
Complete the Lewis Dot diagram
04/18/23
Perchlorate Ion
O 24 valence e–
total 32 valence e–
charge 1 e–
Cl 7 valence e–
Only 8 electronsshown
(4 pairs)
OO Cl
OO
Skeleton diagramLewis Dot diagram
16 e– pairs
••O
• •• •
••••O• •
• •••
••O
• •• •
••
••O• •
• •••
Cl••Cl: 7 - 8/2 - 0 = +3
O: 6 - 2/2 - 6 = -1
total charge = -1
O: 6 - 2/2 - 6 = -1O: 6 - 2/2 - 6 = -1O: 6 - 2/2 - 6 = -1
Why areorganic perchlorate salts
potentially explosive?
04/18/23
Pair ActivityFormal Charges
Develop a procedure to determine the formal charges on a molecule or ion.• Initially, one person should be responsible
for writing all the information. • The other person should be responsible for
saying what should be written.
04/18/23
Pair ActivityFormal Charges
On a card or a piece of paper• write each sequential step necessary to
determine the formal charges.• number each step.• amend and update the list.
Determine the formal charges on
• acetic acid, CH3COOH
• acetate ion, CH3COO–
04/18/23
Acetic Acid and Acetate Ion
O ••••
HCH
H
H
CO
••
••••
O ••••
••
O ••
••
CH
H
H
C
O ••••
CH
H
H
CO
••
••••
C: 4 - 8/2 - 0 = 0
total charge = 0
H: 1 - 2/2 - 0 = 0O: 6 - 4/2 - 4 = 0O: 6 - 4/2 - 4 = 0
C: 4 - 8/2 - 0 = 0
total charge = -1
H: 1 - 2/2 - 0 = 0O: 6 - 2/2 - 6 = -1O: 6 - 4/2 - 4 = 0
04/18/23
Formal Charges
What are the formal charges on the atoms in• ozone• sulfite ion• sulfur trioxide
04/18/23
“Connection” diagram
Convert to aLewis Dot diagram
Carbonate Ion
charge 2 e–
O 18 valence e–
total 24 valence e–
C 4 valence e–
OO
O
C
2-
O ••• •
••••O• •
• •••
••O
• •• •
••
••C
2-
Lewis Dot diagram
12 e– pairsWhich oxygen atomhas the double bond?
ResonanceEdward A. Mottel
Department of Chemistry
Rose-Hulman Institute of Technology
04/18/23
Resonance
C••••
• •• •O
O ••••
•• ••
O
••
••••
••
C••••
• •• •O
O ••••
••••
O
••
••••
••
C••••
• •• •O
O ••••
••
O ••••
••••••
For some skeleton and dot structuresthere is not an unique, correct answer.
Carbonate ion has three resonant configurations.
Individually,none of these structuresrepresents the actual structure.
But the average of thesestructures is a good
representation of the ion.
04/18/23
Resonance
C••••
• •• •O
O ••••
•• ••
O
••
••••
••
C••••
• •• •O
O ••••
••••
O
••
••••
••
C••••• •
• •OO ••••
••
O ••••
••••••
What is the carbon-oxygen bond order?
(1+1+2) / 3 = 1.33
04/18/23
Resonance
C••••
• •• •O
O ••••
•• ••
O
••
••••
••
C••••
• •• •O
O ••••
••••
O
••
••••
••
C••••• •
• •OO ••••
••
O ••••
••••••
Where is the negative charge locatedin each contributing structure?
On the oxygen atoms with a single bond.
04/18/23
Resonance
C••••
• •• •O
O ••••
•• ••
O
••
••••
••
C••••
• •• •O
O ••••
••••
O
••
••••
••
C••••• •
• •OO ••••
••
O ••••
••••••
What is the average formal chargeon each atom in the carbonate ion?
carbon: 0; oxygen: -2/3
04/18/23
Thiocyanate Ion
Only 8 electronsshown
(4 pairs)
total 16 valence e–
charge 1 e–
S 6 valence e–
C 4 valence e–
N 5 valence e–
Skeleton diagrams
NS CNS C
04/18/23
N
• •• •
••••S
• •• •
••C
-•• ••N
• •• •
••S••••C
-•• •• ••NS
• •
•• • •
••••••
Thiocyanate Ion
C-••
Formal charges favor which of these structures?
Possible Lewis Dot structures
0 0 -1 +1 0 -2 -1 0 0
-
A “double headed” arrow is the symbol for resonance.What carbon-sulfur and carbon-nitrogen bond orders
are predicted for this ion?
C-S: 1.5; C-N: 2.5
(nitrogen is slightly more electronegative than sulfur: 3.1, 2.4)
04/18/23
Resonance Structures
Factors to consider when picking the best resonance structures:• Zero or low formal charges are the best.• Formal charges are consistent with the
electronegativity of the elements involved.• Alternating positive and negative formal
charges are better than highly polarized patterns.
04/18/23
C
• •H
C••
••
C••
••
C••H
• •
C••••C••
• • • •
H
•• H
H
H••
••C
••H
C ••
••
C ••
••
C• •
H
••
C •••• C••
••••
H
••H
H
H••
••
Benzene, C6H6
total 30 valence e–
H 6 valence e–
C 24 valence e–
Cyclic structure
Shorthand notation
It is assumed that there are enough hydrogenatoms at each corner to fulfill carbon’s valence.
Even shorter notation
In this structure, what does the circle represent?What is the carbon-carbon bond order?
CyclopentadieneCyclopentadiene reacts with hydroxide ion
to make the cyclopentadienide ion.
H H
+ OH–
H–
+ H2O
One of severalresonance structures
What is the formula of cyclopentadiene?What is the carbon-carbon bond order in C5H5–?
–
What is the shorthand notation for thecyclopentadienide ion?
04/18/23
04/18/23
