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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
not.
C
C
H
O
H
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?
