2. Molecular Orbital Theory (What is it??)  Better bonding model than valence bond theory  Electrons are arranged in “molecular orbitals”  Dealing with valence shell electrons

3. Molecular OrbitalsMolecular Orbitals  Combination of atomic orbitals  2 atomic orbitals------2 molecular orbitals (one bonding, one antibonding)  Molecular region where electrons are likely to be found within a chemical compound  Mathematically based  Behave like molecules  Deals with electrons arranged in molecules, NOT atoms

4. Bonding Molecular Orbital ( σ )  Contain electrons involved in chemical bonding  Contribute to bond strength  Increase stability  Lower energy level for electrons  More electrons present, more stability within molecule  Increased electron density between atoms

5. Anti-bonding Molecular Orbitals ( σ *)  Contain electrons NOT involved in bonding  Electrons hang out away from bond  Decreased stability and bond strength  Higher energy level for electrons  Instability within molecule  Decreased electron density between atoms

6. **Electrons want to be at a LOW energy level SO---generally pair up and reside in bonding molecular orbitals.

7. How are electrons placed in molecular orbitals? 1)Electrons want to be in the lowest-energy molecular orbitals as possible. 2)Only 2 electrons found in each molecular orbital. 3)Electrons are placed in molecular orbitals by themselves (parallel spins) unless they have to be paired up (opposite spins).

8. Bond OrderBond Order  Determined by molecular orbitals = (# electrons in bonding MO) – (# electrons in anti-bonding MO) 2

9. Ex. 1 Molecular orbital energy-level diagram H2H2

10. Example 2: Molecular orbital energy-level diagram H2+H2+

11. What happens when “p” atomic orbitals combine?  Each “p” orbital combines with another “p” orbital—2 molecular orbitals produced  Of the 2 molecular “p” orbitals—  1 lower energy bonding orbital  1 higher energy anti-bonding orbital  One p orbital produces orbital overlap  σ p, σ * p  Other 2 p orbitals overlap in parallel  π p, π* p

12. Example 3: O 2

14. What is polarity?What is polarity?  Focus on covalent bonds  Contributes to the properties of chemical compounds  Based on electronegativity difference between atoms within chemical bonds

15. Electronegativity and Bond Polarity  Covalent Bond Polar Covalent Ionic Bond (0—0.5)(0.5—1.9) (1.9—3.5)

16. Chose your polarityChose your polarity 1)Nonpolar Covalent  small to same electronegativity difference  Occurs among same or similar atoms bonded together  EQUAL sharing of electrons, equal distribution of electron density 2)Polar Covalent  difference in electronegativity  Most electronegative atom pulls electrons towards it  UNEQUAL sharing of electrons, electron density displaced toward electronegative atom

17. Partial ChargesPartial Charges  Exist in polar covalent molecules  Demonstrates tendency of one end to be “slightly” negative or positive  NO charge on the whole molecule

18. Polar vs. Nonpolar Molecules  Polar  Exhibit difference in electronegativity—partial charges  Based on polar covalent bonds  Nonpolar  No electronegativity difference  No partial charges

19. Dipoles  Bond Dipoles  Charge separation within the chemical bond  Only applies to a specific chemical bond  Molecular Dipole  Charge separation among WHOLE molecule  Includes ALL chemical bonds  Equal to bond dipole in a diatomic molecule (ex. HCl, HI)

20. Example 1: HClExample 1: HCl

21. What about polyatomic molecules ?

22. Distinguishing between Polar/Nonpolar Molecules 1)Draw Lewis Structure 2)Identify polar bonds 3)Determine molecular shape 4)Draw dipoles

23. Example 2: CO 2  Polar bonds?  Dipoles cancel?

24. Example 3: H 2 OExample 3: H 2 O  Polar bonds?  Dipoles cancel?

25. Example 4: BF 3  Polar bonds?  Dipoles cancel?

26. Example 5: CCl 4  Polar bonds?  Dipoles cancel?

27. Example 6: CH 3 ClExample 6: CH 3 Cl  Polar bonds?  Dipoles cancel?

28. Example 7: NH 3  Polar bonds?  Dipoles cancel?

29. Homework  pp. 427-429 #25, 29, 37-38, 45-46, 55, 63