1. Bonding – General Concepts

2. Shows how valence electrons are arranged among atoms in a molecule. Reflects central idea that stability of a compound relates to noble gas electron configuration. Lewis Structures

3. C H H H Cl.. Completing a Lewis Structure - CH 3 Cl Add up available valence electrons: C = 4, H = (3)(1), Cl = 7 Total = 14 Join peripheral atoms to the central atom with electron pairs. Complete octets on atoms other than hydrogen with remaining electrons Make carbon the central atom..

4. Multiple Covalent Bonds: Double bonds Two pairs of shared electrons Ethene

5. Multiple Covalent Bonds: Triple bonds Three pairs of shared electrons Ethyne

6. Resonance Resonance is invoked when more than one valid Lewis structure can be written for a particular molecule. The actual structure is an average of the resonance structures. Benzene, C 6 H 6 The bond lengths in the ring are identical, and between those of single and double bonds.

7. Resonance Bond Length and Bond Energy Resonance bonds are shorter and stronger than single bonds. Resonance bonds are longer and weaker than double bonds.

8. Resonance in Ozone, O 3 Neither structure is correct. Oxygen bond lengths are identical, and intermediate to single and double bonds

9. Resonance in a carbonate ion: Resonance in an acetate ion: Resonance in Polyatomic Ions

10. Localized Electron Model Lewis structures are an application of the “Localized Electron Model” L.E.M. says: Electron pairs can be thought of as “belonging” to pairs of atoms when bonding Resonance points out a weakness in the Localized Electron Model.

11. Models Models are attempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world. Models can be physical as with this DNA model Models can be mathematical Models can be theoretical or philosophical

12. Fundamental Properties of Models  A model does not equal reality.  Models are oversimplifications, and are therefore often wrong.  Models become more complicated as they age.  We must understand the underlying assumptions in a model so that we don’t misuse it.

13. VSEPR VSEPR V alence S hell E lectron P air R epulsion theory.V alence S hell E lectron P air R epulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs.Most important factor in determining geometry is relative repulsion between electron pairs. Molecule adopts the shape that minimizes the electron pair repulsions. MOLECULAR GEOMETRY

14. VSEPR – Valence Shell Electron Pair Repulsion X + E Overall Structure Forms 2 LinearAX 2 AX 3 Trigonal PlanarAX 3, AX 2 E 4 TetrahedralAX 4, AX 3 E, AX 2 E 2 5 Trigonal bipyramidalAX 5, AX 4 E, AX 3 E 2, AX 2 E 3 6 OctahedralAX 6, AX 5 E, AX 4 E 2 A = central atom X = atoms bonded to A E = nonbonding electron pairs on A

15. VSEPR: Linear AX 2 CO 2

16. John A. Schreifels Chemistry 211 Chapter 10-16 Figure 9.2 Molecular Shapes 2,3,4 electron pairs.

17. John A. Schreifels Chemistry 211 Chapter 10-17 Figure 9.3 Molecular shapes 5, 6 electron pairs

18. AX 2 - Linear AX - Linear Draw CO 2 and HF

19. AX 3 AX 2 E BF 3 SnCl 2 Triganol planar Bent

20. VSEPR: Tetrahedral AX 4 AX 3 E AX 2 E 2 CCl 4 PCl 3 Cl 2 O tetrahedral Triangular Pyramidal Bent

21. VSEPR: Trigonal Bi-pyramidal AX 5 AX 4 E AX 3 E 2 AX 2 E 3 PCl 5 SF 4 ClF 3 I3-I3-I3-I3- Triangular bipyramidal See-saw T-shaped Linear

22. VSEPR: Octahedral AX 6 AX 5 E AX 4 E 2 SF 6 ICl 4 - BrF 5 Octahedral Square pyramidal Square planar

23. Are the following molecules polar or nonpolar ? Which will dissolve in water? OF 2 SO 3 PCl 3 SF 6 NH 3

24. 24 Formal charge (FC) in Lewis structures Comparison of the formal number of valence electrons about an atom in the Lewis structure of a molecule and comparison of FN with the number of valence electrons (VE) in the neutral atom. Computation of FC: FC = VE (neutral atom) - LE (atom in molecule) - 1/2BE (atom in molecule) where VE = the number of valence electrons in the neutral atom LE = the number of lone pair electrons on the atom in the molecule BE= the number of bonding electrons on the atom in the molecule

25. 25 Example of formal charge computation: ozone, O 3 Note: New charge for molecule = 0, sum of formal charges must = 0. VE (atom)=666 1/2 BE (molecule)=-2-3-1 LE (molecule)=-4-2-6 FC=0+1-1

26. 26 HCN = atomic compositional structure HCN possesses 10 VE = Lewis compositional structures Two possible Lewis constitutional structures: H-C-N or H-N-C Both need to have 10 VE in their Lewis structure Problem: Try to achieve an acceptable Lewis structure (duet and octet rule followed) for both. Isomers: Same composition, two different constitutional Lewis structures

27. 27 HCN = atomic compositional structure HCN: 10 VE = Lewis compositional structures Two possible Lewis constitutional structures H-C-N or H-N-C Any acceptable Lewis structure for HCN needs to show 10 VE Try to achieve an acceptable Lewis structure (duet and octet rules obeyed) for all isomeric structures. Two acceptable Lewis structures. Which is better?

28. 28 Use formal charges to decide on the stability of isomeric Lewis structures VE (atom) 145154 1/2 BE (molecule) -1-4-3 -1-4-3 UE (molecule)00-2 00-2 ________________________________________________________ FC on atom000 0+1-1 Important: the net charge of composition HCN = 0, so the sum of the formal charges in any acceptable Lewis structure must be = 0 also.