Calculating Formal
Charges
Simple but significant
What is the Formal Charge of
an Atom?
• The Formal Charge is a mathematical summation of
the number of actual electrons associated with an
atom in a molecule. “Electronic Bookkeeping”
• The Formal Charge is all determined relative to the
number of valence electrons an atom would have in
the ground state.
• Remember, valence electrons are those found in the
outermost unfilled shell.
What is the Formal Charge of
an Atom?
• Anytime you see a charge in a molecule, it tells you
that the atom with the charge has more or less
electrons than it normally would.
• A -1 charge equals one extra electron on an atom.
• A +1 charge equals one less electron on an atom.
What is the Formal Charge of
an Atom?
• Charges can be found on all sorts of atoms common
to organic chemistry, including carbons, oxygens,
nitrogens and halides.
• Remember that carbon typically has four covalent
bonds. If it has only three, then it will be a charged
atom, depending on whether it has a lone pair or
O
not.
H
C
H
C
What is the Formal Charge of
an Atom?
• Oxygen atoms, when neutral, have two covalent
bonds and two lone pairs, like in water. Often
though oxygen atoms may have negative or positive
charges.
• Nitrogen atoms, when neutral, have three covalent
bonds and one lone pair. These can also have
negative or positive charges, depending on what is
bonded.
What is the Formal Charge of
an Atom?
• The presence of more or less electron density in a
molecule dictates the flow of electrons in a process
so this is really helpful information.
• So – how do you calculate the Formal Charge on an
atom?
What is the Formal Charge of
an Atom?
• The equation to determine the Formal Charge on an
atom:
FC = (The number of valence electrons on the atom in
the ground state) – ½ (total number of electrons in
covalent bonds attached to the atom, with each covalent
bond having two electrons) – (all electrons in lone pairs
on an atom, with each lone pair equal to two electrons).
• So:
FC = #Valence – ½ (bonding) – (all lone pair electrons)
What is the Formal Charge of
an Atom?
• Keep in mind that Formal Charge is for a specific
atom so you have to know exactly which atom you
are doing the calculation for.
• Determine the Formal Charge on the nitrogen atom
in the following:
H
H
N
H
Calculating some Formal
Charges
• Nitrogen is in Group V and has 5 valence electrons
H
H
in the ground state.
N
H
• FC = #Valence – ½ (bonding) – (all lone pair
electrons)
•
FC(N) = 5 – ½ (6) – (2) = 5 – 3 – 2 = 0
• Nitrogen has no charge – its neutral.
Calculating some Formal
Charges
• Calculate the formal charge on oxygen in the
hydronium ion shown:
H
H
O
H
• Oxygen is in Group VI and has 6 valence electrons
in the ground state.
• FC = #Valence – ½ (bonding) – (all lone pair
electrons)
Calculating some Formal
Charges
• FC = #Valence – ½ (bonding) – (all lone pair
electrons)
•
FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1
H
H
O
H
Calculating some Formal
Charges
• Every time you see that positive charge on an atom,
it technically means the atom is missing one entire
electron.
H
H
O
H
• In this case, the oxygen only has five electrons, not
six.
Calculating some Formal
Charges
• Calculate the formal charge on oxygen in the
molecule shown:
O
• Again, oxygen is in Group VI and has 6 valence
electrons in the ground state.
Calculating some Formal
Charges
• FC(O) = 6 – ½ (2) – (6) = 6 – 1 – 6 = -1
• As in this sodium salt:
O Na
Calculating some Formal
Charges
• Calculate the formal charge on oxygen in the
molecule shown:
H
O
• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1
H
O
Calculating some Formal
Charges
• Calculate the formal charge on oxygen in the
molecule shown:
H
O
Calculating some Formal
Charges
• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1
H
O
Calculating some Formal
Charges
• Calculate the formal charge on nitogen in the
molecule shown:
H
N
Calculating some Formal
Charges
• FC(O) = 5 – ½ (8) – (0) = 5 – 4 – 0 = +1
H
N
Calculating some Formal
Charges
• Calculate the formal charge on the
(highlighted in red) in the molecule shown:
O
S
H3C
H
C
H
carbon
Calculating some Formal
Charges
• Carbon is in Group IV.
• FC(O) = 4 – ½ (6) – (2) = 4 – 3 – 2 = -1
O
S
H3C
H
C
H
Calculating some Formal
Charges
• Calculate the formal charge on the
(highlighted in red) in the molecule shown:
O
S
H3C
CH3
sulfur
Calculating some Formal
Charges
• Sulfur is in Group IV.
• FC(O) = 6 – ½ (6) – (2) = 6 – 3 – 2 = +1
O
S
H3C
CH3
Calculating some Formal
Charges
• Final Problem: Calculate the formal charge on the
oxygen (highlighted in red) in the molecule shown:
O
S
H3C
CH3
Calculating some Formal
Charges
• Oxygen is in Group IV.
• FC(O) = 6 – ½ (2) – (6) = 6 – 1 – 6 = -1
O
S
H3C
CH3
Formal Charges
• Just remember: Valence Electrons minus half the
bonding electrons (two per bond) minus all of the
lone pair electrons (two per pair)…
• Thanks for reading…