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Chemical Bonding Set 5

Chemical Bonding Set 5. SAVE PAPER AND INK!!! When you print out the notes on PowerPoint, print "Handouts" instead of "Slides" in the print setup. Also, turn off the backgrounds (Tools>Options>Print>UNcheck "Background Printing")!. Credits.

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Chemical Bonding Set 5

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  1. Chemical BondingSet 5 SAVE PAPER AND INK!!! When you print out the notes on PowerPoint, print "Handouts" instead of "Slides" in the print setup. Also, turn off the backgrounds (Tools>Options>Print>UNcheck "Background Printing")!

  2. Credits • Thank you to Mr. Neil Rapp who provided the bulk of this powerpoint on his website www.chemistrygeek.com • Other information comes from Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

  3. MOLECULAR GEOMETRY

  4. MOLECULAR GEOMETRY Molecule adopts the shape that minimizes the electron pair repulsions. VSEPR • Valence Shell Electron Pair Repulsion theory. • Most important factor in determining geometry is relative repulsion between electron pairs.

  5. Some Common Geometries Linear Tetrahedral Trigonal Planar

  6. VSEPR charts • Use the Lewis structure to determine the geometry of the molecule • Electron arrangement establishes the bond angles • Molecule takes the shape of that portion of the electron arrangement • Charts look at the CENTRAL atom for all data! • Think REGIONS OF ELECTRON DENSITY rather than bonds (for instance, a double bond would only be 1 region)

  7. Other VSEPR charts

  8. Structure Determination by VSEPR Water, H2O The electron pair geometry is TETRAHEDRAL 2 bond pairs 2 lone pairs The molecular geometry is BENT.

  9. Structure Determination by VSEPR Ammonia, NH3 The electron pair geometry is tetrahedral. The MOLECULAR GEOMETRY — the positions of the atoms — is TRIGONALPYRAMIDAL.

  10. You will need to memorize: • Shapes: Angles: • Linear 180 • Bent 104.5 • Trigonal planar 120 • Tetrahedral 109.5 • Trigonal pyramidal 107.3 • Trigonal bipyramidal 2 @ 180, 3 @ 120 • Octahedral 90

  11. Hybridization • A hybrid occurs when two things are combined and the result has characteristics of both • EX: hybrid car (uses gas and electricity) • During chemical bonding, different atomic orbitals undergo hybridization.

  12. Carbon’s Hybridization • Consider methane, CH4 • The carbon atom has four valence electrons with the electron configuration of [He]2s22p2. • You may expect the two unpaired p electrons to bond with other atoms and the two paired s electrons to remain as a lone pair • However, carbon undergoes hybridization, a process in which atomic orbitals mix and form new, identical, hybrid orbitals.

  13. The hybrid orbitals in carbon Note that each hybrid orbital contains one electron that it can share with another atom, giving carbon 4 bonding sites. The hybrid orbital is called an sp3 orbital because the four orbitals form from one s and three p orbitals.

  14. Other Hybridizations Anything with a linear shape is a result of sp hybridization. CO and CO2 are common examples. These often result from double and triple bonds. Anything with a trigonal planar shape is a result of sp2 hybridization. AlCl3 and nitrate ion are examples. Can result from a double bond. Anything with a tetrahedral geometry is a result of sp3 hybridization. Carbon is the most common example.

  15. Lone Pairs & Hybridization • Lone pairs also occupy hybrid orbitals. • Compare the hybrid orbitals of BeCl2 (linear) and H2O (bent). • Why does the water molecule have sp3 orbitals and the BeCl2 has sp? • The two O-H bonds occupy two of the sp3 orbitals in water, and the two lone pairs occupy the other two. • Beryllium doesn’t have any lone pairs, so it’s geometry remains liner with sp hybridization.

  16. Homework Questions 1) Although the VSEPR model is correct in predicting that CH4 is tetrahedral, NH3 is trigonal pyramidal, and H2O is bent, the model in its simplest form does not account for the fact that these molecules do not have exactly the same bond angles (<HCH is 109.5 degrees, as expected for a tetrahedron, but <HNH is 107.3 degrees, and <HOH is 104.5 degrees). Explain these deviations from the tetrahedral angle. Use outside resources if necessary.

  17. More Homework 2) Predict the molecular structure and bond angles for the following: • HCN, PH3, CHCl3 3) Compare the molecular shapes and hybrid orbitals of PF3 and PF5 molecules. Explain why their shapes differ. HINT: PF5 has sp3d hybridization (why would that be?) 4) List, in a table, the Lewis structure, molecular shape, bond angle, and hybrid orbitals for molecules of CS2, CH2O, H2Se, CCl2F2, and NCl3.

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