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.