Dr. Williamson’s Molecular Geometry Notes 1 Molecular Geometry Dr. V.M. Williamson Texas A & M University Student Version Valence Shell Electron Pair Repulsion- VSEPR 1. Valence e- ___________to some extent 2. Electron pairs move as far away as possible to ________________ repulsive forces 3. Shape determined by repulsive position of electron pairs to give base or electron geo. MUST consider both lone pairs and bonds. 4. Molecular geo from base or electron geo ! If no lone pairs (only bonds), then electron geo _____ molecular geo 5. Molecular geometry describes the position of the bonded atoms around the central atom. 2 Principal originator: R. J. Gillespie in the 1950’s VSEPR: Definition of Terms " Central atom: bonded to more than one other atom " Regions of high electron density: bonding electron pair = ___ region (multiple bonds count = ___ region) lone pair of electrons = ___ region " Electronic geometry: geometry of regions of high electron density around central atom " Molecular geometry: geometry of atoms around central atom Determining Geometry 1. Count the regions of electron density from Lewis structure. (lone pairs and single, double or triple bonds are just 1 region.) 2. Determine the "base" or electron geometry. 3. Determine the number of electron regions that are bonded to another atom. 4. Determine the actual molecular geometry from the "base" or electron geometry 4 " To minimize repulsions, situate electron pairs as far apart as possible VSEPR: Electronic Geometries 109.5° 109.5° O VSEPR 6
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Dr. Williamson’s Molecular Geometry Notes
1
Molecular Geometry
Dr. V.M. WilliamsonTexas A & M UniversityStudent Version
Valence Shell Electron Pair Repulsion-VSEPR
1. Valence e- ___________to some extent 2. Electron pairs move as far away as possible
to ________________ repulsive forces3. Shape determined by repulsive position of
electron pairs to give base or electron geo. MUST consider both lone pairs and bonds.
4. Molecular geo from base or electron geo! If no lone pairs (only bonds), then electron geo _____
molecular geo5. Molecular geometry describes the position
of the bonded atoms around the central atom.
2 Principal originator: R. J. Gillespie in the 1950’s
VSEPR: Definition of Terms" Central atom: bonded to more than
one other atom" Regions of high electron density:
bonding electron pair = ___ region (multiple bonds count = ___ region)lone pair of electrons = ___ region
" Electronic geometry: geometry of regions of high electron density around central atom
" Molecular geometry: geometry of atoms around central atom
Determining Geometry1. Count the regions of electron density
from Lewis structure. (lone pairs and single, double or triple bonds are just 1 region.)
2. Determine the "base" or electron geometry.
3. Determine the number of electron regions that are bonded to another atom.
4. Determine the actual molecular geometry from the "base" or electron geometry
4
" To minimize repulsions, situate electron pairs as far apart as possible
Why is water attracted to this comb? (Try it at home J)
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Dr. Williamson’s Molecular Geometry Notes
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Why polarity??
" Solubility!!" See demo" Likes dissolve _____" Ionic and polar covalent substances
dissolve in polar solvents! Think of ionic as polar extreme!!
" Nonpolar substances dissolve in nonpolar solvents
" What will dissolve in water? In CCl4?37
Valence Bond (VB) Theory(orbital overlap model)" A bond is formed by the overlap of an
orbital from 1 atom with an orbital from another atom. This theory allows for mixing of orbitals (hybridization) to fit the shape and energies observed in the molecule.
Principal originator: L. Pauling in the 1930’s & 40’s
" VSEPR = Repulsion between e- pairs such that distance was maximized and effect of repulsion minimized determines _______.
" Valence Bond = atomic orbitals overlap to form bonds. Atomic __________ orbitals can form.
" Both FAILED to explain:! why O2 was attracted to a magnet (was
paramagnetic) ! Or why SO2 had 2 equivalent bond that are an
intermediate between a double and single bond.
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Liquid N2 and O2 with a Magnet
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Molecular Orbital Theory" MO theory = molecular orbital theory.
Assumes that new MOLECULAR orbitals are formed from the atomic orbitals of the bonding atoms. These molecular orbitals can reflect the geom. of the molecule, but are hard to visualize.
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Bonding molecular orbitals (MO’s) = e- spend most of time between the two nuclei.
Antibonding MO’s = e- spend most of time on the outside of nuclei.
Consider H2
Dr. Williamson’s Molecular Geometry Notes
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MO Energy Level Diagrams H2 through N2 NEW filling order
Ener
gy Molecularorbitals
2p 2p
π2p
σ*2p
π*2p
1s 1s σ1s
σ*1s
2s σ2s
σ*2s
Atomic orbitals
2s
σ2p
MO Energy Level Diagrams O2 through Ne2 NEW filling order