Molecular Geometry Molecular Geometry Problems with

CHEM 107, Spring 2015
Molecular Geometry
Class #20
Molecular Geometry
CHEM 107
L.S. Brown
Texas A&M University
•  What shapes are molecules?
•  Can we actually measure them? How?
•  How can we explain and predict these
shapes?
Problems with Orbital Overlap
Idea
Methane
•  Simple overlap of atomic orbitals
suggests that molecules with p valence
electrons should have bond angles
of 90°
•  Experiments show other bond angles
are much more common
•  What about a molecule like methane?
Why CH4 and not CH2?
•  All 4 bonds are equivalent: same length,
same energy
•  H-C-H bond angles 109.5°
•  Can’t reconcile this with overlap of s, p
orbitals
•  “Hybrid Orbitals”
Hybrid Orbitals
Hybrid Orbitals
•  “Patch” to the orbital overlap model.
Needed to match observed facts.
•  For CH4, need to form 4 identical bonds
on carbon
•  Need 4 identical orbitals, each with one
electron
•  “Create” sp3 hybrid orbitals
•  Imagine mixing 2 orbitals. Easiest pair is an s
and a single p
+"
!
–"
!
•  2 new ‘sp hybrid’ orbitals from 2 original
orbitals
© 2015, L.S. Brown
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CHEM 107, Spring 2015
sp3 Hybrid Orbitals
sp3 Hybrid Orbitals
•  Mix 4 orbitals, get 4 new identical
orbitals
•  Energy between s and p
•  Angles of 109.5°
•  Tetrahedral shape
•  Let’s us explain bonding in methane
(and lots of other molecules, too)
NH3
•  Lewis Structure:
•  Single bonds to 4 other atoms
CH4, NH4+, (CH3)4Si
•  Other combinations with “steric number”
of four, including “lone pairs” of e–’s
H2O, NH3
NH3 - Trigonal Pyramid
H
N
H
H
•  N: Bound to 3 atoms, 1 Lone Pair
•  Steric Number = 3 + 1 = 4
•  need 4 orbitals: sp3 hybrids
H 2O
•  sp3 orbitals point at tetrahedral angles
•  3 orbitals form bonds, 1 holds lone pair
•  Shape defined by atoms, NOT lone
pairs
•  “Trigonal pyramid”
H2O - Bent Shape
•  Lewis Structure:
H
O
H
•  O: Bound to 2 atoms, 2 Lone Pairs
•  Steric Number = 2 + 2 = 4
•  Need 4 orbitals: sp3 hybrids
•  sp3 orbitals point at tetrahedral angles
•  2 orbitals form bonds, 2 hold lone pairs
•  Shape defined by atoms, NOT lone
pairs
•  “Bent” molecule
© 2015, L.S. Brown
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CHEM 107, Spring 2015
Other Hybridizations
Steric Number & Hybridization
•  We can form other hybrids
s + 3p’s ! sp3
s + 2p’s ! sp2
s + p ! sp
•  Same ideas, but get orbitals at different
angles
Orbital Orientation vs.
Molecular Geometry
•  Molecular shape based on position of
ATOMS
•  All hybrids used for bonds ! molecular
geometry same as orbital orientation
•  Lone pairs on central atom ! molecular
shape differs from orbital orientation
Steric Number of 3
•  sp2 hybrids, trigonal planar orientation
•  No lone pairs ! trigonal planar
molecule
BF3, NO3
–
•  One lone pair ! bent triatomic
molecule
O3, NO2
Steric Number of 2
•  sp hybrids, linear orientation
•  No lone pairs ! linear molecule
CO2
Steric Number of 4
•  sp3 hybrids, tetrahedral orientation
•  No lone pairs !tetrahedral
CH4, NH4+
•  One lone pair ! trigonal pyramid
NH3
•  Two lone pairs ! bent
H 2O
© 2015, L.S. Brown
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CHEM 107, Spring 2015
Steric Number of 5
Steric Number of 5
•  One lone pair ! “seesaw” molecule
•  Trigonal bipyramid orientation
•  No lone pairs ! trigonal bipyramid
molecule
SF4
•  Two lone pairs ! T-shape molecule
ClF3
PCl5
•  Lone pairs: Positions not all equivalent!
•  Three lone pairs ! Linear molecule
I3–
Steric Number of 6
Multiple Bonds
•  Octahedral orientation
•  No lone pairs ! octahedral
•  How can orbitals overlap to form double
or triple bonds?
•  Start with a simple example: C2H4
•  First draw a Lewis structure
SF6
•  One lone pair ! square pyramid
ClF5
•  Two lone pairs ! square planar
XeF4
Ethylene - C2H4
Ethylene - C2H4
H
H
C
H
H
C
H
C
H
•  Carbons have steric number of 3 !
•  Overlap of sp2 orbitals from each
carbon forms single bond.
•  Double bond?
H
sp2
C
H
•  Double bond?
•  “Sideways” overlap of unhybridized p
orbitals from carbon atoms
•  “π bond”
© 2015, L.S. Brown
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CHEM 107, Spring 2015
Acetylene - C2H2
Acetylene - C2H2
H
H
C
C
H
•  Carbons have steric number of 2 ! sp
•  “Sigma bond” from
overlap of 2 sp hybrids
•  Triple bond?
C
C
H
•  Triple bond ! 3 bonds
•  Need 2 π bonds in addition to σ bond
•  “Sideways” overlap of unhybridized p
orbitals on carbon atoms, this time
using 2 p orbitals from each carbon (px
and py )
Multiple Bonds
•  In orbital overlap (or “localized bond”)
model:
– Single bond is a σ bond
– Double bond consists of a σ bond and
a π bond
– Triple bond consists of a σ bond and
two π bonds
© 2015, L.S. Brown
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