1. Formal Charge & Resonance Structures These ARE NOT Cornell Notes

2. Learning Objectives Determine whether a Lewis structure is plausible by calculating formal charges. Explain why resonance occurs, and identify resonance structures.

3. 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)

4. Question Is there a way to determine whether our Lewis structure is plausible?

5. 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-)

6. Examples

7. Formal Charge Usually, the most plausible Lewis structure is one with formal charges of zero on all atoms.

8. 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.

9. Formal Charge Adjacent atoms in a structure should not carry formal charges of the same sign.

10. 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.

11. Example #1 (p174) Draw a Lewis structure for NCl 3. Calculate the formal charges on each atom.

12. Example #2 (p174) A student has proposed two condensed structural formulas—H 2 NOH and H 2 ONH—for a compound with the molecular formula H 3 NO. Write a Lewis structure corresponding to each formula, assign formal charges, and select the more plausible Lewis structure.

13. Practice #1 (p 175) Draw a Lewis structure for CH 2 O. Calculate the formal charges on each atom.

14. Practice #2 (p175) Draw the Lewis resonance structures for NO 2 -

15. 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.

16. 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.

17. Example #1 Draw the Lewis resonance structures for NO 2 -

18. Example #2 Draw the Lewis resonance structures for SO 2.

19. Practice Draw the Lewis resonance structures for O 3.