Lecture 8.3- VSEPR

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Section 8.3 Lecture for Honors & Prep Chemistry

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  • Methane is a tetrahedral molecule. The hydrogens in methane are at the four corners of a regular tetrahedron, and the bond angles are all 109.5°. Interpreting Diagrams How do the resulting H—C—H bond angles compare to the tetrahedral angle?
  • An ammonia molecule is pyramidal. The unshared pair of electrons repels the bonding pairs.
  • This comparison of water and carbon dioxide illustrates how unshared pairs of electrons can affect the shape of a molecule made of three atoms. a) The water molecule is bent because the two unshared pairs of electrons on oxygen repel the bonding electrons. b) In contrast, the carbon dioxide molecule is linear. The carbon atom has no unshared electron pairs.
  • This comparison of water and carbon dioxide illustrates how unshared pairs of electrons can affect the shape of a molecule made of three atoms. a) The water molecule is bent because the two unshared pairs of electrons on oxygen repel the bonding electrons. b) In contrast, the carbon dioxide molecule is linear. The carbon atom has no unshared electron pairs.
  • Shown here are common molecular shapes.
  • Lecture 8.3- VSEPR

    1. 1. Bellwork Pipe cleaner shapes # of “legs” picture name 2 3 4
    2. 2. The Valence-shell electron-pair repulsion (VSEPR) Model states that electron pairs in a molecule will be as far apart as they can be because they repel each other. Linear Trigonal planar Tetrahedral
    3. 3. An electron domain is an area around an atom where electrons are found.
    4. 4. An electron domain is an area around an atom where electrons are found. They can be bonding or non-bonding (lone pairs).
    5. 5. An electron domain is an area around an atom where electrons are found. They can be bonding or non-bonding (lone pairs). Bonding domains can contain two, four, or six e - .
    6. 6. The number of electron domains determines the electron-domain geometry.
    7. 7. The number of electron domains determines the electron-domain geometry. The number of lone pairs determines the final shape of the molecule.
    8. 9. <ul><ul><ul><li>The hydrogens in a methane molecule are at the four corners of a tetrahedron. </li></ul></ul></ul><ul><ul><ul><li>The bond angles are 109.5°. </li></ul></ul></ul>
    9. 10. <ul><ul><ul><li>The measured H—N—H bond angle in ammonia is only 107°. </li></ul></ul></ul>
    10. 11. <ul><ul><ul><li>The measured bond angle in water is about 105°. </li></ul></ul></ul>Lone Pair Bonding Pair
    11. 12. Lone pairs repel more than bonding electrons
    12. 13. Water’s bond angle is 104.5 ° (much less than the tetrahedral angle of 109.5°) because its lone pairs need extra room.
    13. 15. <ul><ul><ul><li>A carbon dioxide molecule is linear. </li></ul></ul></ul>
    14. 16. <ul><ul><ul><li>Nine Possible Molecular Shapes </li></ul></ul></ul>
    15. 17. <ul><ul><li>VSEPR theory enables prediction of 3-dimensional molecular shape because the valence electron pairs </li></ul></ul><ul><ul><ul><li>are attracted to each other. </li></ul></ul></ul><ul><ul><ul><li>form molecules with only four possible shapes. </li></ul></ul></ul><ul><ul><ul><li>stay as far apart as possible. </li></ul></ul></ul><ul><ul><ul><li>always form tetrahedral shapes. </li></ul></ul></ul>
    16. 18. Bellwork- VSEPR Theory Fill in the missing information 3D shape molecule Lewis structure # of e - domains ( bonding areas ) picture name methane CH 4 ammonia NH 3 water H 2 O
    17. 19. Bellwork- VSEPR Theory Fill in the missing information 3D shape molecule Lewis structure # of e - domains ( bonding areas ) picture name methane CH 4 4 ammonia NH 3 4 water H 2 O 4
    18. 20. Bellwork- VSEPR Theory Fill in the missing information 3D shape molecule Lewis structure # of e - domains ( bonding areas ) picture name methane CH 4 4 tetrahedral ammonia NH 3 4 trigonal pyramidal water H 2 O 4 bent

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