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Chapter 9  Molecular Geometry and Bonding Theories CHEMISTRY   The Central Science  9th Edition David P. White
<ul><li>Lewis structures give atomic connectivity: they tell us which atoms are physically connected to which. </li></ul><...
Molecular Shapes
<ul><li>In order to predict molecular shape, we assume the valence electrons repel each other.  Therefore, the molecule ad...
 
<ul><li>There are five fundamental geometries for molecular shape: </li></ul>Molecular Shapes
 
<ul><li>When considering the geometry about the central atom, we consider all electrons (lone pairs and bonding pairs). </...
<ul><li>To determine the shape of a molecule, we distinguish between lone pairs (or non-bonding pairs, those not in a bond...
 
 
<ul><li>To determine the electron pair geometry: </li></ul><ul><ul><li>draw the Lewis structure, </li></ul></ul><ul><ul><l...
VSEPR Model
<ul><li>The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles </li></ul><ul><li>We determine the electron p...
<ul><li>The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles </li></ul><ul><li>By experiment, the H-X-H bo...
<ul><li>The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles </li></ul>VSEPR Model
<ul><li>The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles </li></ul><ul><li>Similarly, electrons in mul...
<ul><li>Molecules with Expanded Valence Shells </li></ul><ul><li>Atoms that have expanded octets have AB 5  (trigonal bipy...
 
 
<ul><li>Molecules with Expanded Valence Shells </li></ul><ul><li>To minimize e   e   repulsion, lone pairs are always p...
Molecules with Expanded Valence Shells VSEPR Model
<ul><li>Shapes of Larger Molecules </li></ul><ul><li>In acetic acid, CH 3 COOH, there are three central atoms. </li></ul><...
<ul><li>When there is a difference in electronegativity between two atoms, then the bond between them is polar. </li></ul>...
Molecular Shape and Molecular Polarity
<ul><li>In water, the molecule is not linear and the bond dipoles do not cancel each other. </li></ul><ul><li>Therefore, w...
 
<ul><li>The overall polarity of a molecule depends on its molecular geometry. </li></ul>Molecular Shape and Molecular Pola...
Molecular Shape and Molecular Polarity
<ul><li>Lewis structures and VSEPR do not explain why a bond forms. </li></ul><ul><li>How do we account for shape in terms...
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Molecular Geometry Slide 1 Molecular Geometry Slide 2 Molecular Geometry Slide 3 Molecular Geometry Slide 4 Molecular Geometry Slide 5 Molecular Geometry Slide 6 Molecular Geometry Slide 7 Molecular Geometry Slide 8 Molecular Geometry Slide 9 Molecular Geometry Slide 10 Molecular Geometry Slide 11 Molecular Geometry Slide 12 Molecular Geometry Slide 13 Molecular Geometry Slide 14 Molecular Geometry Slide 15 Molecular Geometry Slide 16 Molecular Geometry Slide 17 Molecular Geometry Slide 18 Molecular Geometry Slide 19 Molecular Geometry Slide 20 Molecular Geometry Slide 21 Molecular Geometry Slide 22 Molecular Geometry Slide 23 Molecular Geometry Slide 24 Molecular Geometry Slide 25 Molecular Geometry Slide 26 Molecular Geometry Slide 27 Molecular Geometry Slide 28 Molecular Geometry Slide 29 Molecular Geometry Slide 30

Molecular Geometry

  1. 1. Chapter 9 Molecular Geometry and Bonding Theories CHEMISTRY The Central Science 9th Edition David P. White
  2. 2. <ul><li>Lewis structures give atomic connectivity: they tell us which atoms are physically connected to which. </li></ul><ul><li>The shape of a molecule is determined by its bond angles. </li></ul><ul><li>Consider CCl 4 : experimentally we find all Cl-C-Cl bond angles are 109.5  . </li></ul><ul><ul><li>Therefore, the molecule cannot be planar. </li></ul></ul><ul><ul><li>All Cl atoms are located at the vertices of a tetrahedron with the C at its center. </li></ul></ul>Molecular Shapes
  3. 3. Molecular Shapes
  4. 4. <ul><li>In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimized this repulsion. </li></ul><ul><li>We call this process V alence S hell E lectron P air R epulsion ( VSEPR ) theory. </li></ul><ul><li>There are simple shapes for AB 2 and AB 3 molecules. </li></ul>Molecular Shapes
  5. 6. <ul><li>There are five fundamental geometries for molecular shape: </li></ul>Molecular Shapes
  6. 8. <ul><li>When considering the geometry about the central atom, we consider all electrons (lone pairs and bonding pairs). </li></ul><ul><li>When naming the molecular geometry, we focus only on the positions of the atoms. </li></ul>Molecular Shapes
  7. 9. <ul><li>To determine the shape of a molecule, we distinguish between lone pairs (or non-bonding pairs, those not in a bond) of electrons and bonding pairs (those found between two atoms). </li></ul><ul><li>We define the electron domain geometry by the positions in 3D space of ALL electron pairs (bonding or non-bonding). </li></ul><ul><li>The electrons adopt an arrangement in space to minimize e  -e  repulsion. </li></ul>VSEPR Model
  8. 12. <ul><li>To determine the electron pair geometry: </li></ul><ul><ul><li>draw the Lewis structure, </li></ul></ul><ul><ul><li>count the total number of electron pairs around the central atom, </li></ul></ul><ul><ul><li>arrange the electron pairs in one of the above geometries to minimize e  -e  repulsion, and count multiple bonds as one bonding pair. </li></ul></ul>VSEPR Model
  9. 13. VSEPR Model
  10. 14. <ul><li>The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles </li></ul><ul><li>We determine the electron pair geometry only looking at electrons. </li></ul><ul><li>We name the molecular geometry by the positions of atoms. </li></ul><ul><li>We ignore lone pairs in the molecular geometry. </li></ul><ul><li>All the atoms that obey the octet rule have tetrahedral electron pair geometries. </li></ul>VSEPR Model
  11. 15. <ul><li>The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles </li></ul><ul><li>By experiment, the H-X-H bond angle decreases on moving from C to N to O: </li></ul><ul><ul><li>Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. </li></ul></ul><ul><ul><li>Therefore, the bond angle decreases as the number of lone pairs increase. </li></ul></ul>VSEPR Model
  12. 16. <ul><li>The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles </li></ul>VSEPR Model
  13. 17. <ul><li>The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles </li></ul><ul><li>Similarly, electrons in multiple bonds repel more than electrons in single bonds. </li></ul>VSEPR Model
  14. 18. <ul><li>Molecules with Expanded Valence Shells </li></ul><ul><li>Atoms that have expanded octets have AB 5 (trigonal bipyramidal) or AB 6 (octahedral) electron pair geometries. </li></ul><ul><li>For trigonal bipyramidal structures there is a plane containing three electrons pairs. The fourth and fifth electron pairs are located above and below this plane. </li></ul><ul><li>For octahedral structures, there is a plane containing four electron pairs. Similarly, the fifth and sixth electron pairs are located above and below this plane. </li></ul>VSEPR Model
  15. 21. <ul><li>Molecules with Expanded Valence Shells </li></ul><ul><li>To minimize e   e  repulsion, lone pairs are always placed in equatorial positions. </li></ul>VSEPR Model
  16. 22. Molecules with Expanded Valence Shells VSEPR Model
  17. 23. <ul><li>Shapes of Larger Molecules </li></ul><ul><li>In acetic acid, CH 3 COOH, there are three central atoms. </li></ul><ul><li>We assign the geometry about each central atom separately. </li></ul>VSEPR Model
  18. 24. <ul><li>When there is a difference in electronegativity between two atoms, then the bond between them is polar. </li></ul><ul><li>It is possible for a molecule to contain polar bonds, but not be polar. </li></ul><ul><li>For example, the bond dipoles in CO 2 cancel each other because CO 2 is linear. </li></ul>Molecular Shape and Molecular Polarity
  19. 25. Molecular Shape and Molecular Polarity
  20. 26. <ul><li>In water, the molecule is not linear and the bond dipoles do not cancel each other. </li></ul><ul><li>Therefore, water is a polar molecule. </li></ul>Molecular Shape and Molecular Polarity
  21. 28. <ul><li>The overall polarity of a molecule depends on its molecular geometry. </li></ul>Molecular Shape and Molecular Polarity
  22. 29. Molecular Shape and Molecular Polarity
  23. 30. <ul><li>Lewis structures and VSEPR do not explain why a bond forms. </li></ul><ul><li>How do we account for shape in terms of quantum mechanics? </li></ul><ul><li>What are the orbitals that are involved in bonding? </li></ul><ul><li>We use Valence Bond Theory: </li></ul><ul><ul><li>Bonds form when orbitals on atoms overlap. </li></ul></ul><ul><ul><li>There are two electrons of opposite spin in the orbital overlap. </li></ul></ul>Covalent Bonding and Orbital Overlap
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