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Molecular Structure: the VSEPR Model. Valence Shell electron-pair repulsion VSEPR model Useful to predict geometry of a molecule Postulate: The structure around an atom is determined by minimizing electron pair repulsions (both bonding and nonbonding). . . . . BeCl 2.
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Molecular Structure: the VSEPR Model • Valence Shell electron-pair repulsion VSEPR model • Useful to predict geometry of a molecule • Postulate: The structure around an atom is determined by minimizing electron pair repulsions (both bonding and nonbonding)
BeCl2 Angle = 180o Cl Cl Be 2 electron pairs: Best arrangement is Linear
F Angle = 120o B F F 3 electron pairs: Best arrangement is Trigonal Planar with 120o angles
H 4+4=8 electrons C H H Angle = 109.5o H 4 effective electron pairs: Best arrangement is tetrahedral Angle = 109.5o Electron arrangement:tetrahedral Geometry: tetrahedral
N H H H The tetrahedral arrangement of electron pairs around the nitrogen atom in the ammonia molecule.Molecular geometry is trigonal pyramidal, not tetrahedral. 5+3=8 electrons
The tetrahedral arrangement of the four electron pairs around oxygen in the water molecule. The V-shaped molecular structure of the water molecule. 6+2=8 electrons O H H
In a bonding pair of electrons, the electrons are shared by two nuclei. In a lone pair, both electrons must be close to a single nucleus and tend to take up more of the space around that atom
PCl5 Cl Cl Angle = 90o P Cl Angle = 120o Cl Cl 5 electron pairs: Best arrangement is trigonal bipyramid
Molecular structure of PCl6- (and SF6) Octahedral Geometry
Molecule Geometry: Central Atoms without Lone Pairs
.. .. .. .. _ I I I .. .. .. .. .. Central Atom with Lone Pairs Three possible arrangements of the electron pairs in the I3- ion. axial equatorial Electron arrangement: Trigonal bipyramid Geometry: Linear To minimize electron pair repulsion place the lone pairs in equatorial, not axial positions
electron-pair arrangement and geometry of XeF4 F F 6 effective electron pairs Xe F F Octahedral electron arrangement Geometry: Square Planar
Summary of 5 e-pairs 5 effective pairs: 1 lone pair- See-Saw 5 effective pairs: 2 lone pairs T-Shaped 5 effective pairs: 3 lone pairs Linear
Summary of 6 e-pairs 6 effective pairs: 1 lone pair- Square Pyramidal 6 effective pairs: 2 lone pairs Square Planar
O N O O Multiple Bonds • A multiple bond counts as one effective electron pair NO3- _ 3 effective electron pairs: Best arrangement is trigonal planar with 120o
• Note: angles of 120o or more do not result in appreciable bond distortion: e.g. SO2 (18 valence electrons) Compare to CO2 Linear, nonpolar SO2, bent, polar S O O 3 effective electron pairs: Best electron arrangement is trigonal planar with 120o Geometry is BENT with 1200 (also known as V-shaped or Angular)
More than one central atomMethanol: CH3OH H H H C O H
VSEPR Does not always workIn the end the experiment determines the geometry Example is PH3 phosphine: P H H Actual Angle is 94o Not as predicted by VSEPR (like NH3 ~ 107o) H