vsepr theory (molecular shapes) - magadh university · ii. valence shell electron pair repulsion...
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VSEPR Theory (Molecular Shapes)
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INTRODUCTION
A) Lewis structures do not indicate the three dimensional shape of a molecule. They do not show the arrangement space of the atoms, what we call the molecular geometry or molecular structure.
B) Molecules have definite shapes and the shape of a molecule controls some of its chemical and physical properties.
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II. Valence Shell Electron Pair Repulsion Theory - VSEPR - predicts the shapes of a number of molecules and polyatomic ions.
A) Assumptions of VSEPR Theory
1) Electron pairs in the valence shell of an atom tend to orient themselves so that the total energy is minimized. This means that: the electrons will approach the nucleus as close as possible yet take positions as far away from each other as possible to minimize _______________ .
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2) Because lone pairs of electrons are spread out more broadly than bond pairs, repulsions are greatest between two lone pairs, intermediate between a lone pair and a bond pair, and weakest between two bonding pairs of electrons.3) Repulsive forces decrease rapidly with increasing interpair angle - greatest at 90o, much weaker at 120o, and very weak at 180o.
B) What are the ideal arrangements of electron pairs to minimize repulsions?
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1) We need to identify the number of regions of high electron density, i.e. the number of stereoactive sets of electrons. One stereoactive set can be a single, double or triple bond.
2) If only single bonds, the number of stereoactive sets is equal to the bond pairs plus the lone pairs of electrons around the central atom.
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# of Lone Pairs+
# of Bonded Atoms Hybridization Examples
2
3
4
5
6
sp
sp2
sp3
sp3d
sp3d2
BeCl2
BF3
CH4, NH3, H2O
PCl5
SF6
How do I predict the hybridization of the central atom?
Count the number of lone pairs AND the numberof atoms bonded to the central atom
10.4
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10.4
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Valence shell electron pair repulsion (VSEPR) model:
Predict the geometry of the molecule from the electrostatic repulsions between the electron (bonding and nonbonding) pairs.
AB2 2 0
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
10.1
linear linear
B B
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Cl ClBe
2 atoms bonded to central atom
0 lone pairs on central atom
10.1
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AB2 2 0 linear linear
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0trigonal planar
trigonal planar
10.1
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10.1
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AB2 2 0 linear linear
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0trigonal planar
trigonal planar
10.1
AB4 4 0 tetrahedral tetrahedral
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10.1
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AB2 2 0 linear linear
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0trigonal planar
trigonal planar
10.1
AB4 4 0 tetrahedral tetrahedral
AB5 5 0trigonal
bipyramidaltrigonal
bipyramidal
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10.1
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AB2 2 0 linear linear
Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0trigonal planar
trigonal planar
10.1
AB4 4 0 tetrahedral tetrahedral
AB5 5 0trigonal
bipyramidaltrigonal
bipyramidal
AB6 6 0 octahedraloctahedral
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10.1
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10.1
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bonding-pair vs. bondingpair repulsion
lone-pair vs. lone pairrepulsion
lone-pair vs. bondingpair repulsion> >
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB3 3 0trigonal planar
trigonal planar
AB2E 2 1trigonal planar bent
10.1
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB3E 3 1
AB4 4 0 tetrahedral tetrahedral
tetrahedral trigonal pyramidal
10.1
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
AB4 4 0 tetrahedral tetrahedral
10.1
AB3E 3 1 tetrahedraltrigonal
pyramidal
AB2E2 2 2 tetrahedral bent
HO
H
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
10.1
AB5 5 0trigonal
bipyramidaltrigonal
bipyramidal
AB4E 4 1trigonal
bipyramidaldistorted
tetrahedron
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
10.1
AB5 5 0trigonal
bipyramidaltrigonal
bipyramidal
AB4E 4 1trigonal
bipyramidaldistorted
tetrahedron
AB3E2 3 2trigonal
bipyramidal T-shaped
ClF
F
F
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
10.1
AB5 5 0trigonal
bipyramidaltrigonal
bipyramidal
AB4E 4 1trigonal
bipyramidaldistorted
tetrahedron
AB3E2 3 2trigonal
bipyramidal T-shaped
AB2E3 2 3trigonal
bipyramidal linear
I
I
I
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
10.1
AB6 6 0 octahedraloctahedral
AB5E 5 1 octahedralsquare
pyramidal
Br
F F
FF
F
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Class
# of atomsbonded to
central atom
# lonepairs on
central atomArrangement of electron pairs
MolecularGeometry
VSEPR
10.1
AB6 6 0 octahedraloctahedral
AB5E 5 1 octahedralsquare
pyramidal
AB4E2 4 2 octahedralsquare planar
Xe
F F
FF
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10.1
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