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Predicting Molecular Geometry with VSEPR Theory

Learn how to predict the geometry of molecules using VSEPR theory. Derive Lewis structures, understand electron pair repulsion, identify central atoms, and determine molecular shapes. Examples and homework included.

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Predicting Molecular Geometry with VSEPR Theory

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  1. CHEMISTRY 161 Chapter 10 Chemical Bonding II www.chem.hawaii.edu/Bil301/welcome.html

  2. PREDICTING THE GEOMETRY OF MOLECULES H O H 1. derive Lewis structure of the molecule 2. discriminate between bonding and non-bonding electron pairs 3. VALENCE SHELL ELECTRON PAIR REPULSION

  3. VALENCE SHELL ELECTRON PAIR REPULSION VSEPR 1. identify in a compound the central atom 2. electrons repel each other 3. valence electron pairs stay as far apart as possible 4. non-bonding electrons repel more than bonding electrons

  4. central atom non-bonding pairs no non-bonding pairs

  5. AB2 BeCl2 Be Cl Cl TWO ELECTRON PAIRS AROUND BERYLLIUM ATOM

  6. Cl Be Cl 180° 270° Be Be 90° 180° LINEAR ARRANGEMENT BEST IT PUTS ELECTRON PAIRS FURTHEST APART

  7. AB3 BF3 F F B F THREE ELECTRON PAIRS AROUND THE BORON ATOM

  8. F F B F 120° 120° B 120° THREE ELECTRON PAIRS AROUND THE BORON ATOM TRIGONAL PLANAR ARRANGEMENT BEST

  9. B MOLECULAR SHAPE F F F THE SHAPE OF BF3 IS TRIGONAL PLANAR.

  10. AB4 H H C H H 90° 90° C 90° 90° CH4 four electron pairs expect square planar

  11. C better arrangement for four electron pairs TETRAHEDRAL 109.5° bigger than 90 ° in square planar tetrahedral 4 electron pairs put on the H-atoms

  12. C C TETRAHEDRAL H 109.5° H H H shape of CH4 is tetrahedral

  13. AB5 F F F P F F P PF5 FIVE ELECTRON PAIRS AROUND PHOSPHORUS 5 electron pairs trigonal bipyramidal

  14. Bond angle F 900 F P F P F 1200 F shape of PF5 is trigonal bipyramidal two of the F atoms different from the others

  15. AXIAL Bond angle F 900 F F P F 1200 EQUATORIAL F

  16. AB6 SF6 six electron pairs around the sulfur atom F F F S S F F F octahedral 6 electron pairs

  17. 900 F F F S S F F F shape of SF6 is octahedral

  18. central atom non-bonding pairs no non-bonding pairs

  19. AB2E AB3 SeO2 O Se O

  20. O Se O Se VSEPR treats double bonds like a single bond THREE ELECTRON PAIRS AROUND SELENIUM ELECTRON PAIR GEOMETRY TRIGONAL PLANAR

  21. Se Se O O ADD OXYGENS SeO2 IS V-SHAPED (OR BENT) THE MOLECULAR SHAPE IS THE POSITION OF THE ATOMS

  22. AB3E AB4 H N H NH3 H electron pairs around the nitrogen atom

  23. N H N H NH3 H PUT ON THE 3 H ATOMS N H H H NH3 is trigonal pyramidal

  24. AB2E2 AB4 four electron pairs around the oxygen atom H O H PUT ON THE 2 H-ATOMS O O H H shape of H2O is V-shaped or bent

  25. S AB4E AB5 SF4 F F F S F TRIGONAL BIPYRAMID

  26. F F F S F S F F F F WHERE DOES LONE PAIR GO? OR lone pairs occupy the trigonal plane (the “equator”) to minimize the number of 90° repulsions

  27. AB4E AB3E2 AB2E3 SF4 1 lone pair See-saw shaped ClF3 2 lone pairs T-shaped XeF2 3 lone pairs Linear F F F F Xe S Cl F F F F F F lone pairs occupy the trigonal plane (the “equator”) first to minimize the number of 90° repulsions

  28. AB5E F F F Br F F Br AB6 BrF5 Square pyramidal

  29. AB6 AB4E2 F F F Xe F F : F Xe Xe F : F F Xe F F F XeF4 lone pairs MUST BE AT 1800

  30. Summary of Molecular Shapes Total valence electron pairs Electron Pair Geometry Lone electron pairs Shape of Molecule 2 Linear 0 Linear 0 Trigonal planar Trigonal planar 3 1 V-shaped 0 Tetrahedral 4 Tetrahedral 1 Trigonal pyramid 2 V-shaped

  31. Total valence electron pairs Electron Pair Geometry Lone electron pairs Shape of Molecule 0 Trig. bipyramid. 1 See-saw Trigonal bipyramidal 5 2 T-shaped 3 Linear 0 Octahedral 6 Octahedral 1 Square pyramid 2 Square planar

  32. POLYATOMICS molecules with no single central atom we apply our VSEPR rules to each atom in the chain Example: ETHANOL

  33. H H H C C O H H H ETHANOL C2H5OH The atoms around the carbons form a. tetrahedral arrangement The atoms around the oxygen form a V-shaped structure.

  34. H O H H C C H H H

  35. EXAMPLES BF4- ICl4- Cl2O SO2Cl2 Cl2CO Cl2SO N2F2 NH4+ NH2OH

  36. Homework Chapter 10 pages 367-377, problem sets

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