Lecture 25: VSEPR. Reading: Zumdahl 13.13 Outline Concept behind VSEPR Molecular geometries. VSEPR Background. • The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding, but what about geometry?.
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• The Lewis Dot Structure approach provided some
insight into molecular structure in terms of bonding,
but what about geometry?
3D structure is determined by minimizing repulsion of electron pairs.
• Must consider both bonding and lone pairs in
• Example: NH3 (both bonding and lone pairs).
• The previous examples illustrate the strategy for
applying VSEPR to predict molecular structure:
1. Construct the Lewis Dot Structure
2. Arranging bonding/lone electron pairs
in space such that repulsion is minimized.
• Linear Structures: angle between bonds is 180°
• Example: BeF2
• Trigonal Planar Structures: angle between bonds is 120°
• Example: BF3
• Pyramidal: Bond angles are <120°, and structure
• Example: NH3
• Tetrahedral: angle between bonds is ~109.5°
• Example: CH4
• Tetrahedral: angle may vary from 109.5° exactly due to size differences between bonding and lone pair electron densities
• Classic example of tetrahedral angle shift from 109.5° is water:
• Comparison of CH4, NH3, and H2O:
• Trigonal Bipyramidal, 120° in plane, and two orbitals at 90° to plane:
• Example, PCl5:
• Octahedral: all angles are 90°:
• Example, PCl6:
• Square Planar versus “See Saw”
No dipole moment
• Driving force for last structure was to
maximize the angular separation of the lone pairs.
• VSEPR and resonance structures. Must look at
VSEPR structures for all resonance species to
predict molecular properties.