Lecture 25: VSEPR Reading: Zumdahl 13.13 Outline –Concept behind VSEPR –Molecular geometries.

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Presentation transcript:

Lecture 25: VSEPR Reading: Zumdahl Outline –Concept behind VSEPR –Molecular geometries

VSEPR Background Recall from last lecture that we had two types of electron pairs: bonding and lone. The Lewis Dot Structure approach provided some insight into molecular structure in terms of bonding, but what about geometry? Valence Shell Electron Pair Repulsion (VSEPR). 3D structure is determined by minimizing repulsion of electron pairs.

VSEPR Background (cont.) Example: CH 4 Must consider both bonding and lone pairs in minimizing repulsion. Lewis Structure VSEPR Structure

VSEPR Background (cont.) Example: NH 3 (both bonding and lone pairs). Lewis Structure VSEPR Structure

VSEPR Applications 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.

VSEPR Applications Linear Structures: angle between bonds is 180° Example: BeF 2 180°

VSEPR Applications Trigonal Planar Structures: angle between bonds is 120° Example: BF 3 120°

VSEPR Background (cont.) Pyramidal: Bond angles are <120°, and structure is nonplanar: Example: NH 3 107°

VSEPR Applications Tetrahedral: angle between bonds is ~109.5° Example: CH °

VSEPR Applications Tetrahedral: angle may vary from 109.5° exactly due to size differences between bonding and lone pair electron densities bonding pair lone pair

VSEPR Applications Classic example of tetrahedral angle shift from 109.5° is water:

VSEPR Applications Comparison of CH 4, NH 3, and H 2 O:

VSEPR Applications Trigonal Bipyramidal, 120° in plane, and two orbitals at 90° to plane: Example, PCl 5 : 90° 120°

VSEPR Applications Octahedral: all angles are 90°: Example, PCl 6 : 90°

Advanced VSEPR Applications Square Planar versus “See Saw” See Saw Square Planar No dipole moment

Advanced VSEPR Applications Driving force for last structure was to maximize the angular separation of the lone pairs.

Advanced VSEPR Applications VSEPR and resonance structures. Must look at VSEPR structures for all resonance species to predict molecular properties.

VSEPR Applications Provide the Lewis dot and VSEPR structures for CF 2 Cl 2. Does it have a dipole moment? 32 e - Tetrahedral