CH101 Lec6

CH101 Lec6 - CHAPTER 6: MOLECULAR STRUCTURE Lewis...

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Section 6.1 CHAPTER 6: MOLECULAR STRUCTURE Lewis structures - 2D representations of 3D molecules. Valence Shell Electron Pair Repulsion ( VSEPR ) –  A molecule adopts a geometry that minimizes the  repulsion between the bonding and lone pairs of  electrons Electrons repel other electrons. ..they want to get as  far apart from each other as possible.
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Applying VESPR Draw Lewis structure Count the number of electron regions around the central atom. Multiple bonds count as one electron region. The arrangement of electron pairs that minimizes repulsion is called the electron group geometry The arrangement of atoms is called the molecular geometry
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Atoms obeying the octet rule have two, three or four electron groups (bonding electrons or lone pairs) LINEAR TRIGONAL PLANAR TETRAHEDRAL Electron group geometry
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2 atoms and/or lone pairs around the central atom Ex. CO (carbon monoxide): C O C O 180 ° 180 ° Bond Angle 180 ° LINEAR C O O 180 o
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H C O H Section 6.1 3 atoms and/or lone pairs around the central atom Ex. H CO (acetone): H C O H 120 ° 120 ° 120 ° TRIGONAL PLANAR
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Section 6.1 4 atoms and/or lone pairs around the central atom 109 ° out in TETRAHEDRAL
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Summary of VSEPR-electron group geometry TRIGONAL PLANAR 3 atoms and/or lone pairs 180 ° LINEAR 109 ° out in TETRAHEDRAL 4 atoms and/or lone pairs C O 180 ° 180 ° H C O H 120 ° 120 ° H H C H H 109 ° 120 °
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Lone pair vs. bonding pair repulsion Relative strengths of the interactions are lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair
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Section 6.1 Molecular Shape 1. Determine electron group geometry by considering  both bonding and lone pair electrons 2. Determine molecular shape by considering the  positions of the atoms only 3. Molecular shape cannot be determined correctly if  the electron group geometry is not considered first
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Electron group geometry: Linear Molecular shape: Linear O C O = = LINEAR, 180 ° C O N N LINEAR, 180 °
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S O O N O H BENT, 120 ° 2 atoms and  1 lone pair  around central atom Electron group geometry: Trigonal planar Molecular shape: Bent or Trigonal planar F B F F O C H H TRIGONAL PLANAR,  120 ° 3 atoms around the central  atom
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Electron group geometry: Tetrahedral Molecular Shape: Pyramidal, Tetrahedral or Bent PYRAMIDAL, 109 ° 3 atoms and  1 lone pair  around the central atom N H H H O H H H +1 TETRAHEDRAL, 109 ° H C H H H F Si F F F S H H O H H BENT, 109 ° 2 atoms and  2 lone pairs  around central atom
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Section 6.1 Molecular Shape- Examples P Cl Cl Cl Pyramidal, ~109 ° (electron geometry:tetrahedral) Trigonal planar, 120 ° (electron geometry: trigonal planar) O C O O -2 N O Cl NOCl Bent, 120 ° (electron geometry: trigonal planar)
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These classifications can be used to describe both electron group geometry and molecular shape. Expanded Octets
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This note was uploaded on 03/29/2008 for the course CH 101 taught by Professor Bigham during the Fall '08 term at N.C. State.

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CH101 Lec6 - CHAPTER 6: MOLECULAR STRUCTURE Lewis...

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