1. Lecture 3

2. Resonance forms When several possible Lewis structures with multiple bonds exist, all of them should be drawn (the actual structure is an average)

3. Expanded Shells There are two situations where expanded shells are invoked. 1. A conventional electron bonding diagram without expansion can be drawn which obeys the octet rule but results in large positive charge build-up on the central atom. The build-up is reduced by using the d orbitals on the central atom to receive electrons. The reduction in the number of electrons around the S gives a higher Z* causing the 3d orbitals to become more stable. They now receive electrons from the oxygen lone pairs. Lone pair on sulfur holding two electrons. Empty 3d orbital on S Electrons move from lone pair into 3d orbital to yield structures above.

4. Expanded Shells 2. If there are five or more atoms bonded to the central atom then the octet rule is violated. The invocation of d orbitals is necessary. 10e around P Only for elements from 3rd row and heavier, which can make use of empty d orbitals See also: L. Suidan et al. J. Chem. Ed. 1995, 72, 583.

5. Similar examples, now with donation into the central atom without expansion of valence shell. B and Be do not obey octet rule (can provide more stabilizing) ability by receiving electrons.

6. Formal charge Apparent electronic charge of each atom in a Lewis structure Formal charge = (# valence e - in free atom) - (# unshared e - on atom) -1/2 (# bonding electrons to atom) Total charge on molecule or ion = sum of all formal charges Uses of Formal Charges: Favored structures provide minimum formal charges place negative formal charges on more electronegative atoms imply smaller separation of charges

7. Examples: Assign All non-bonding electrons to the atom on which they are found Half of the bonding electrons to each atom in the charge Favored structure provides minimum formal charges places negative formal charges on more electronegative atoms implies smaller separation of charges

8. Problem cases - expanded shells - generating charge to satisfy octets

9. Formal charges and expanded shells As before, some molecules have satisfactory Lewis structures with octets but better ones are obtained with expanded shells. Expansion allows a atom having a negative charge to donate into a positive atom, reducing the charges.

10. Favored structures Obey the octet rule, maximum stabilization for electrons provide minimum formal charges place negative formal charges on more electronegative atoms imply smaller separation of charges

11. Valence shell electron pair repulsion (VSEPR) theory (a very approximate but very useful way of predicting molecular shapes) Electrons in molecules appear in groups. A group is a lone pair, single bond, double Bond, or triple Bond. Each group of electrons repels all other groups Molecules adopt geometries with electron pairs as far from each other as possible those regions where the groups are located are called electron domains the steric number is the number of electron domains

12. Basic molecular shapes for AB 2 and AB 3.

13. Let’s review basic molecular shapes. Observe that the electrons in the Bonds are as far apart as they can be to minimize repulsion. AB n

14. Removing atoms from one basic geometry generates other shapes pyramidal bent tetrahedral

15. The geometries of electron domains. A domain is a lone pair, A single bond, a double A double bond A triple bond.

16. Molecular geometries First useful to classify domains Non-bonding domain A lone pair, Bonding domains A single bond, A double bond A triple bond.

17. Note that lone pairs adopt equatorial positions

18. Molecular geometries

19. Similar for higher steric numbers (=domains)

20. Lone pairs are larger than bonding pairs

21. Effect of lone pairs on molecular geometry

22. Draw Lewis structure Determine electron-domain geometry Determine molecular geometry taking into account preferred sites for lone pairs and multiple bonds (multiple bonds tend to occupy the same positions as lone pairs) In SN=5 (tbp) lp adopt eq.positions; SN=6 (Oct.) lp adopt trans positions SF 4 SO 2 Cl 2 To predict the shapes of molecules with VSEPR model:

23. Boiling Points and Hydrogen bonding

25. Hydrogen bonding in ice The density of water decreases when it freezes and that determines the geology and biology of earth

26. Hydrogen bonding is crucial in biological systems Secondary structure of proteins DNA replication

27. Electronegativity Scales The ability to attract electrons within a chemical, covalent bond Pauling: polar bonds have higher bond strengths. Electronegativity assigned to each element such that the difference of electronegativities of the atoms in a bond can predict the bond strength.

28. Oxidation number/state A “purely ionic” formalism Oxidation number  is the charge that an atom would have if the more electronegative atom in a bond “takes” the 2 electrons Oxidation state is the physical state of the element corresponding to its oxidation number We will use both terms interchangeably NO 3 - 5+ 2- SO 4 2- 6+ 2-

29. Molecular shape and polarity Electronegativity is the ability of an atom to attract electrons from a neighboring atom to which it is bonded Bond polarity measures charge separation between bonded atoms (measured as dipole moment) (increases with increasing difference in electronegativity) In a polyatomic molecule, the dipole moment depends on polarity of bonds and molecular shape Consider each bond dipole as a vector, the molecular dipole moment will be the sum of all vectors Molecules with larger bond dipoles will have smaller angles, eg. PX 3 Electronegativity: F > Cl > Br > I Bond angles: I > Br > Cl > F - Careful with molecules containing hydrogen - Consider EN of central atom as well as its size (e.g. 3.4)

30. The VSEPR model allows many useful qualitative predictions but fails at accurately predicting bond angles and other parameters Better predictions are made from Valence Bond Theory but today Molecular Orbital Theory dominates We will need symmetry and group theory before we study MO theory

31. Try problems 3-1 to 3-17 See also Brown, LeMay, Bursten Chemistry: The Central Science, 10th Ed.

32. Symmetry and group theory

33. Natural symmetry in plants

34. Symmetry in animals

35. Symmetry in the human body

36. The platonic solids

37. Symmetry in modern art M. C. Escher

38. Symmetry in arab architecture La Alhambra, Granada (Spain)

39. Symmetry in baroque art Gianlorenzo Bernini Saint Peter’s Church Rome

40. Symmetry in Native American crafts