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Public Service Announcements

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.

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Public Service Announcements

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  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).

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