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Molecular shapes
A lot of balls... and sticks
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Learning objectives
Describe underlying
principles that govern
theories of molecular
shapes
Use Lewis dot
diagrams to predict
shapes of molecules
using VSEPR
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Valence shell electron pair repulsion
In order to understand properties like
polarity, we need to predict molecular
shapes
Lewis dot structure provides 2D sketch of
the distribution of the valence electrons
among bonds between atoms and lone
pairs; it provides no information about the
shape of the molecule
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A hierarchy of models
VSEPR
Consider the problem in terms of electrostatic repulsion
between groups of electrons (charge clouds, domains)
Valence bond theory
Acknowledges the role of orbitals in covalent bonding
Molecular orbital (MO) theory (the “real” thing)
Accommodates delocalization of electrons - explains
optical and magnetic properties
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Counting sheep clouds
Valence shell electron pair repulsion (VSEPR)
Identify all groups of charge on central atom only:
• Non-bonding pairs count 1
• All bonded atoms count 1 (singles/doubles/triples)
Distribute them about central atom to minimize potential
energy (equals maximum separation)
This specifies electronic geometry (also known as
electron domain geometry or sometimes, confusingly,
as molecular geometry)
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Electronic geometry and molecular
shape
Electronic geometry includes all
atoms and lone pairs on central
atom
H2O has tetrahedral electronic
geometry (2 +2 = 4)
Molecular shape (geometry)
ignores lone pairs
H2O is bent (2 atoms)
Must know electronic geometry
to obtain correct molecular
shape
With no lone pairs: molecular
shape = electronic geometry
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Blob counting: Choices are limited
Groups (domains) of charge range from 2 – 6
Only one electronic geometry for each number
Note: more than one molecular shape follows from electronic geometry depending on number of lone pairs
One surprise: lone pairs occupy more space than the bonded atoms (with very few exceptions) Manifested in bond angles (examples follow)
Molecular shape selection (particularly in trigonal bipyramid – the tricky one)
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Two groups: linear
Except for BeX2 (Be violates octet rule), all cases
with two groups involve multiple bonds
Electronic geometry = molecular shape = linear
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Three groups: trigonal planar
Two possibilities for central atoms with complete octets: Trigonal planar (H2CO)
Bent (SO2)
BCl3 provides example of trigonal planar with three single bonds B is satisfied with 6
electrons – violates octet rule
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Four groups: tetrahedral
Three possibilities: No lone pairs (CH4) -
tetrahedral
One lone pair (NH3) – trigonal pyramid
Two lone pairs (H2O) – bent
Lone pairs need space: • H-N-H angle 107°
• H-O-H angle 104.5°
• Tetrahedral angle 109.5°
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Representations of the tetrahedron
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Five groups of charge: trigonal
bipyramid – most variations Two different positions:
Three equatorial
Two axial
Equatorial positions are lower energy: Lone pairs need more space
Lone pairs require occupy equatorial sites preferentially
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Five bonds, no lone pairs
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Four bonds, one lone pair
Lone pair dictates geometry: equatorial position
has lower energy than axial
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Three bonds, two lone pairs
Both lone pairs occupy equatorial positions –
lower energy than in axial
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Two bonds, three lone pairs
The trend continues: all equatorial positions filled –
lowest energy
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Octahedron has six identical
positions and high symmetry
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No lone pairs
High symmetry
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One lone pair All positions are equally probable
Symmetry reduced
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Two lone pairs
Minimum energy has axial symmetry, lone pairs lie
along straight line
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Molecules with multiple centers
A central atom is any atom with more than one atom
bonded to it
Perform exercise individually for each atom
Electronic geometry and molecular shape will refer only to
the atoms/lone pairs immediately attached to that atom
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Taking it to the next level:
acknowledging orbitals
VSEPR is quite successful in predicting
molecular shapes based on the simplistic
Lewis dot approach
But our understanding of the atom has the
electrons occupying atomic orbitals
How do we reconcile the observed shapes
of molecules with the atomic orbital picture
of atoms