Formal Charge &
Resonance Structures
These ARE
NOT Cornell
Notes
Learning Objectives
• Determine whether a Lewis structure is
plausible by calculating formal charges.
• Explain why resonance occurs, and identify
resonance structures.
Review
• What is electronegativity? (see notes on page
127 of notebook)
• What trend does electronegativity follow in
the periodic table? (see labeled periodic table
on page 128 of notebook)
Question
• Is there a way to determine whether our Lewis
structure is plausible?
Formal Charge
• Formal charge is the difference between the
number of valence electrons in a free (uncombined)
atom and the number of valence electrons assigned
to that atom when bonded to others in a Lewis
structure.
Formal charge = (valence e-) – (# of bonds) – (non-bonding e-)
Examples
Formal Charge
• Usually, the most plausible Lewis structure is one
with formal charges of zero on all atoms.
Formal Charge
• Where non-zero formal charges are required,
they should be as small as possible, and
negative formal charges should appear on the
most electronegative atoms.
Formal Charge
• Adjacent atoms in a structure should not carry
formal charges of the same sign.
Formal Charge
• The total of formal charges on the atoms in a
Lewis structure must be zero for a neutral
molecule and must equal the net charge for a
polyatomic ion.
Example #1 (p174)
Draw a Lewis structure for NCl3. Calculate the
formal charges on each atom.
Example #2 (p174)
A student has proposed two condensed structural
formulas—H2NOH and H2ONH—for a compound with
the molecular formula H3NO. Write a Lewis structure
corresponding to each formula, assign formal charges,
and select the more plausible Lewis structure.
Practice #1 (p 175)
• Draw a Lewis structure for CH2O. Calculate the
formal charges on each atom.
Practice #2 (p175)
Draw the Lewis resonance structures for NO2-
Resonance
• Using the same sequence of atoms, it is
possible to have more than one correct Lewis
structure when a molecule or polyatomic ion
has both a double and a single bond.
Resonance
• Resonance is a situation in which a molecule
or ion can be represented by two or more
plausible Lewis structures that differ only in
the distribution of electrons.
• The true structure is a composite of all
possible Lewis structures.
• The different plausible structures are called
resonance structures.
Example #1
Draw the Lewis resonance structures for NO2-
Example #2
Draw the Lewis resonance structures for SO2.
Practice
Draw the Lewis resonance structures for O3.