1. Resonance • A molecule or polyatomic ion for which 2 or more dot formulas with the same arrangement of atoms can be drawn is said to exhibit RESONANCE.

2. Resonance Example • CO32- • 3 resonance structures can be drawn for CO32- • the relationship among them is indicated by the double arrow. • the true structure is an average of the 3.

3. Resonance Example • CO32- • 3 resonance structures can be drawn for CO32- • the relationship among them is indicated by the double arrow. • the true structure is an average of the 3.

4. Resonance Example • CO32- • 3 resonance structures can be drawn for CO32- • the relationship among them is indicated by the double arrow. • the true structure is an average of the 3

5. Resonance Structures • Another way to represent this is by delocalization of bonding electrons: • (the dashed lines indicate the 4 pairs of bonding electrons are equally distributed among 3 C-O bonds; unshared electron pairs are not shown) • See p. 256

6. VSEPR valence shell electron pair repulsion

7. Molecular Shape • Lewis structures (electron dot structures) show the structure of molecules…but only in 2 dimensions (flat). • BUT, molecules are 3 dimensional! • for example, CH4 is:

8. Molecular Shape • Lewis structures (electron dot structures) show the structure of molecules…but only in 2 dimensions (flat). • BUT, molecules are 3 dimensional! • but in 3D it is: a tetrahedron! = coming out of page = going into page = flat on page

9. Why do molecules take on 3D shapes instead of being flat? • Valence Shell Electron Pair Repulsion theory • “because electron pairs repel one another, molecules adjust their shapes so that the valence electron pairs are as far apart from another as possible.”

10. Why do molecules take on 3D shapes instead of being flat? • Valence Shell Electron Pair Repulsion theory • Remember: both shared and unshared electron pairs will repel one another. Non-Bonding Pairs H—N — H — Bonding Pairs H

11. 5 Basic Molecule Shapes • Linear • Example: CO2

12. 5 Basic Molecule Shapes • Bent or angular • Example: H2O • Notice electron pair repulsion

13. 5 Basic Molecule Shapes • tetrahedral • example: CH4

14. 5 Basic Molecule Shapes • Pyramidal • Example: NH3 • (note: unshared pair of electron repels, but is not considered part of overall shape; no atom there to contribute to the shape)

15. 5 Basic Molecule Shapes • Trigonal planar or planar triangular • Example: BF3

16. Geometry and polarity • Three shapes will cancel out polarity. • Shape One: Linear

17. Geometry and polarity • Three shapes will cancel out polarity. • Planar triangles 120º

18. Geometry and polarity • Three shapes will cancel out polarity. • Tetrahedral

19. Geometry and polarity • Others don’t cancel • Bent

20. Geometry and polarity • Others don’t cancel • Trigonal Pyramidal