1 Recap Resonance When two or more Lewis structures are possible for a molecule it is said to exhibit resonance. This is indicated by double headed arrows:  The (one) actual structure is an average of possible Lewis structures. Example: HCO 3 -

Recap Lecture 7 2 Elements of Period 3 (eg P, S, Cl) have their valence electrons in the third shell The third shell can hold a maximum of 18 electrons in the s, p & d sub-shells Consequently we see structures that may have more than 8 electrons associated with these atoms

3 Example: POCl 3 Total number of valence electrons: (3 x 7) = 32 Oxygen has a valency of 2 so, when uncharged, forms two bonds.

4 Example: POCl 3 P now has 10 electrons, but this is OK as they are in the third shell (max 18 electrons). O has the two bonds expected from its valency. These are resonance structures (only position of the electrons have moved) but not ‘equivalent’. Right hand structure is a better representation of the real structure.

5 Valency and charge Valency works well for C, N, O & F in neutral molecules. For period 3 elements (and below) it is only part of the story, eg S forms compounds with 2, 4 or 6 bonds. If a charge is present, the atom may have more/less bonds than expected.

6 Predicting the best structure Draw the Lewis structure, including lone pairs and multiple bonds. Check for the presence of ‘equivalent’ resonance structures. Check number of bonds on C, N, O & F as a prompt for ‘non-equivalent’ resonance structures. Choose best structure and check again for resonance structures.

7 Chlorite ion, ClO 2 - Total number of valence electrons: 7 + (2 x 6) + 1 = 20 Note: 8 electrons on oxygen remains fixed, Cl has 8, 10 or 12 electrons

8 Sulfate ion, SO 4 2- Total number of valence electrons: 6 + (4 x 6) + 2 = 32 These are all resonance structure but the last one is ‘best’ so far.

9 SO 4 2-, possible structures:

10 Sulfate ion, SO 4 2- There are also other resonance structures related to some of these structures. And the real structure is best represented by an average of these six structures. All S-O bonds are the same length.

By the end of this lecture, you should: −be able to construct Lewis structures of molecules and polyatomic ions containing period 3 elements −recognise situations where resonance occurs −identify non-equivalent resonance structures −be able to choose the resonance structure(s) that are likely to most closely represent the real structure −be able to complete the worksheet (if you haven’t already done so…) 11 Learning Outcomes:

12 Questions to complete for next lecture: 1.Draw the Lewis structures of NH 2 -, NH 3 and NH 4 + and indicate how the number of bonds relates to the valency of nitrogen. 2.Draw the Lewis structure of PCl 3 and PCl 5 and indicate the number of electrons associated with the phosphorus atom. 3.Draw the ‘best’ Lewis structure of SO 2 and SO 3 and give a reason for your choice. 4.Draw the ‘best’ Lewis structure of HPO 3 2- and give a reason for your choice (note H is bonded to the P).