Public Service Announcements

1. Public Service Announcements • Today—begin discussion on bonding and shape…how they’re related • Why is methane actually tetrahedral, not planar? • Why is water bent, not linear • Orbitals…how they ‘morph’ from atomic to molecular • Polarity—how to extend bond polarity to an entire molecule

2. Polar Molecules and Dipole Moments • A polar bond (Chapter 9) has separate centers of positive and negative charge. • A molecule with separate centers of positive and negative charge is a polar molecule. • The dipole moment (m) of a molecule is the product of the magnitude of the charge (d) and the distance (d) that separates the centers of positive and negative charge. m = dd • A unit of dipole moment is the debye (D). • One debye (D) is equal to 3.34 x 10–30 C m.

3. Example 10.4 Explain whether you expect the following molecules to be polar or nonpolar. (a) CHCl3(b) CCl4 Example 10.5 A Conceptual Example Of the two compounds NOF and NO2F, one has m = 1.81 D and the other has m = 0.47 D. Which dipole moment do you predict for each compound? Explain.

4. Bond Dipoles and Molecular Dipoles • A polar covalent bond has a bond dipole; a separation of positive and negative charge centers in an individual bond. • Bond dipoles have both a magnitude and a direction (they are vector quantities). • Ordinarily, a polar molecule must have polar bonds, BUT … polar bonds are not sufficient. • A molecule may have polar bonds and be a nonpolar molecule – IF the bond dipoles cancel.

5. Net dipole Bond Dipoles and Molecular Dipoles • CO2 has polar bonds, but is a linear molecule; the bond dipoles cancel and it has no net dipole moment (m = 0 D). No net dipole • The water molecule has polar bonds also, but is an angular molecule. • The bond dipoles do not cancel (m = 1.84 D), so water is a polar molecule.

6. Molecular Shapes and Dipole Moments To predict molecular polarity: • Use electronegativity values to predict bond dipoles. • Use the VSEPR method to predict the molecular shape. • From the molecular shape, determine whether bond dipoles cancel to give a nonpolar molecule, or combine to produce a resultant dipole moment for the molecule. Note: Lone-pair electrons can also make a contribution to dipole moments.

7. How do bonds actually form? • So far, we’ve only covered that bonds are formed when atoms share (or transfer) an electron(s). • The space electrons occupy—orbitals. • Does our view of atomic orbitals mesh with VSEPR? • Let’s have a look…for H2 bonding…no problem

8. Orbital shapes…spherical, p’s? • S orbitals…no problem, p-orbitals? Recall, dumbbell shape—oriented 90 ° along x, y, z axes • What about something simple like F2? • HCl?? Again, no problem--even though it’s s and p orbital…

9. But those are simple molecules • What about non-linear molecules? • CH4? • Ammonia • Water. • None are 90 °, • P-orbitals ARE • All 90 degrees But these angles are > Ninety degrees…how Is that possible?

10. Hybridization…that’s how! • Energy gap between s and p orbitals is low • An electron absorbs energy and is promoted (2p)

11. Promotion—followed by hybridztn • Remember that promotion is energy intensive, takes energy…BUT…not as much energy saved when you can form additional bonds • Making bonds RELEASES energy, breaking takes E

12. Consistent with what we’ve learned? • Yes…remember that orbitals are mathematical ‘equations’—probability of finding an electron • When you combine several orbitals, the eq changes

13. This helps explain methane • Also important to remember that the number of atomic orbitals INTO a hybrid scheme MUST equal the number of hybrid orbitals OUT of that scheme • Previous example means we’re dealing with an sp3 orbital (one s and three p orbitals).

14. Not all hybrids are bonding orbitals • In each case…methane, ammonia, and water are all sp3 hybridized, but lone pairs of electrons occupy some of these orbitals

15. What about other schemes • Yep, can do those too. What about an sp2 scheme (one s and two p orbitals)? • 3 in, 3 out, all at 120° (just like Electron group geometry) • One orbital isn’t

16. sp Hybridization in Be … with two unused p orbitals. Two AOs combine to form … … two hybrid AOs …

17. Let’s relate EGG to hybrid geo! • Note that the EGG and the hybrid orbital geometry are the same (which is why it’s often necessary to assign EGG in the first place).