About Formal Charges
CHEM 330 handout
The valence electrons of an atom in the bonded state (= one that is part of a molecule)
often cancel out its nuclear charge. Positive (nuclear) and negative (electronic) charges
are then electrostatically balanced. In some bonded states, however, an atom may find
itself surrounded by a greater or a fewer number of valence electrons than expected on
the basis of its nuclear charge. An electrostatic imbalance then occurs.
If the atom in question finds itself with more valence electrons than expected, then it will
be negatively charged. The atom will probably be inclined to behave as an electron
donor, and this propensity will govern much of its reactivity.
If the atom in question finds itself with fewer valence electrons than expected, then it will
be positively charged. The atom will probably be inclined to behave an electron
acceptor, and again this proclivity will control much of its reactivity.
Because organic reactions are movements of electrons, and because electrons always
move from electrostatically negative sites to positive ones, it is essential to know whether
the bonded state of an atom has created an electrostatic imbalance. This will allow us to
rationalize and predict the reactivity of such an atom, and – by extension – of the
molecule containing that atom.
Enter the formal charge
The formal charge of an atom is a parameter that indicates whether
the atom in question is electrostatically balanced or unbalanced.
The formal charge of an atom in a molecule is easily calculated from the complete Lewis
structure of the molecule (i.e., one that shows all bonding and nonbonding electron pairs).
One simply needs to count how many valence electrons (both bonding and nonbonding)
the atom in question has contributed to the molecule, i.e., how many valence electrons are
formally present around the atom, then decide whether the number of such valence
electrons corresponds to an electrostatic balance or an imbalance
The following examples illustrate how this is done.
