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 stratgey for
applying VSEPR to predict molecular structure:
1. Construct the Lewis Dot Structure
2. Arranging bonding/lone electron pairs
in space such that repulsions are minimized.
• Shorthand notation:
1. Refer to central atom as “A”
2. Attached atoms are referred to as “X”
3. Lone pairs are referred to as “E”
H2O: AX2E2 CH4: AX4
BF3: AX3 PCl5: AX5
NH3: AX3E ClF3: AX3E2
• Linear Structures (AX2): angle between bonds is 180°
• Example: BeF2
• Trigonal Planar Structures (AX3): angle between bonds is 120°
• Example: BF3
• Pyramidal (AX3E): Bond angles are <120°, and structure is nonplanar due to repulsion lone-pair.
• Example: NH3
• Tetrahedral (AX4): 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 (AX2E2):
• Comparison of CH4 (AX4), NH3 (AX3E), and H2O (AX2E2):
• Trigonal Bipyramidal (AX5), 120° in plane, and two orbitals at 90° to plane.
• Example, PCl5:
• Octahedral (AX6): all angles are 90°.
• Example, SF6:
• See-Saw versus Square Planar
AX4E: See Saw
AX4E2: Square Planar
• Square Planar versus “See Saw”
No dipole moment
• Driving force for last structure was to
maximize the angular separation of the lone pairs.
Same effect occurs in I3- (AX2E3):
D. square planar