1
Resonance Structures
The more correct way to do Lewis
Dot Structures (book method)
1. Get the sum of all valence electrons from
all atoms. Ignore which electrons came
from which atom.
2. Arrange the elements
3. Place the electrons anywhere in the
compound to satisfy the octet and duet rule.
Counting Valence Electrons
Lone pair electrons belong entirely to
the atom in question.
Shared electrons are divided equally
between the two sharing atoms .
(Valence electrons)
assigned
=
( # lone pair electrons) +
1/2 (# of shared electrons).
Formal Charge
To determine the formal charge on each
atom, take the number of valence
electrons assigned to the atom in the
molecule and subtract if from the number
of valence electrons on th e free, neutral
atom.
Formal charge = valence e lectrons of the
free atom- valence electrons assigned
Rules
The sum of the formal charges of all
atoms in a given molecule or ion must
equal the overall charge on that species.
If different Lewis structures exist
for a species, those with formal charges
on all atoms closest to z ero are the
best.
All negative formal charges should be
on the most electronegative atoms.
Formal charge
Most molecules have a formal charge of 0
on all atoms.
H 1 electron on free – 1 assigned = 0
O 6 electrons on free – 6 assigned = 0
HClO The one on the right is correct Ions
For ions, an electron is added for each
negative charge and an electron is
subtracted for each positive charge
from the total valence electrons.
All ions will have a formal charge on at
least one atom
Na
+
would subtract 1 electron for the +1
SO
42-
would add two electrons for the 2-
