Chemical Bonding II - Maine-Endwell Middle School Bonding II...Chemical Bonding II Molecular...
Transcript of Chemical Bonding II - Maine-Endwell Middle School Bonding II...Chemical Bonding II Molecular...
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Chemical Bonding II
Molecular Geometry
Valence Bond Theory
Phys./Chem. Properties
Quantum Mechanics
Sigma & Pi bonds
Hybridization
MO theory
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Molecular Geometry
3-D arrangement of atoms
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VSEPR
Valence-shell electron-pair
repulsion theory.
All valence shell e- pairs
(or e- “domains”) repel each other.
(Coulomb’s law as applied to the
repulsion of valence electrons.)
Determines the geometry of
e- domains around central atom.
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VSEPR Theory: Rules
1.Single, double, triple bonds and
lone pairs are treated as one e-
domain (approximation)
2.Apply VSEPR to any one
resonance structure
3.Geometry– move e- domains as
far apart as possible in 3-D space.
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AB2 (with no lone pairs)
Beryllium chloride BeCl2
— Be—
Cl—Be—Cl
180o
linear geometry
e-
atoms
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AB3 (with no lone pairs)
Boron trifluoride BF3
B
F
FF
Trigonal planar
120o
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C
AB4 (with no lone pairs)
Methane CH4
tetrahedron
109.5o
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AB5 (with no lone pairs)
Phosphorus Pentachloride PCl5
trigonal bipyramid
equatorial
axial90o
120o
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SF
F
F
F
F
F
AB6 (with no lone pairs)
Sulfur hexafluoride SF6
octahedral
All angles
90o & 180o
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Quiz: Name That Shape!
AB2
AB3
AB4
AB5
AB6
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VSEPR: Lone Pairs
Molecules in which the
central atom has lone pair(s)
ABxEy
central atom
surrounding atoms
lone pairs on A
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ABxEy
Geometry is similar to ABx, but
nonbonding pairs are treated like
bonding pairs to determine
geometry of the e- domains.
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VSEPR
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Bonding e- take up less space
than nonbonding e-
Example: water
Bonding e- are
“focused” between
the nuclei.
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VSEPR
lone pair lone pair bonding pair
vs vs vs
lone pair bonding pair bonding pair
repulsion repulsion repulsion
Decreasing e- pair repulsion
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AB2E
Sulfur dioxide SO2
O S—O
OSO angle < 120o
Draw Lewis structure
S
O O3 e- domains
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Shape: e- Pairs vs. Atoms
S
O O
e- domains are trigonal planar
S
O O
molecule is bent linear
(always state molecular shape)
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17e- tetrahedral trigonal pyramidal
AB3E
Ammonia H—N—H
H
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AB2E2
Water H—O—H
e- tetrahedral molecule: bent linear
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Comparison
AB4 AB3E AB2E2
Methane ammonia water
109.5 107.3 104.5
CH
H
H
H
NH
H
HO
H
H
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AB4E
Sulfur tetrafluoride SF4
S
F F
F FPredict: trigonal bipyramid
for e- domains.
Draw Lewis structure
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AB4E: SF4
trigonal bipyramid: 2 choices
F
F
F
F
F
F
F
F
distorted tetrahedron or seesaw
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AB3E2
ClF3 F
F
F
molecule: T-shaped
A little weird.
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AB2E3Three possibilities for I3
-
Want nonbonding e-
domains farthest apart.
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AB2E3
I3-
molecule: linear
I
I
-
Lone pairs always go on
equatorial position(s)
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AB5E
BrF5
molecule: square pyramidal
FF
FF
F
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AB4E2: 2 Possibilities
Want
nonbonding
e- pairs
farthest
apart.
1
2
XeF4
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AB4E2
XeF4
molecule: square planar
F
FF
F
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You Try It
Predict geometry and
approximate bond angles for:
AlCl4-
XeF2
XeOF2
Text: Tables 10.1 and 10.2.
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Bond Polarity
FH
Shift of e- density toward F,
Thus HF is polar.
d+ d-
F is more
electronegative
than H.
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Overall Molecular Polarity
O C O
Even though CO2 has polar bonds,
it is nonpolar since the individual
bond polarities add to zero.“Dipoles” are a vector quantities.
Symmetrical = nonpolar
“pull test”
(Recall the “Regents” rule about
lone pairs on the central atom.)
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Molecular Polarity
Molecule GeometryDipole
Strength
HF Linear 1.92
HBr Linear 1.08
Water Bent 1.87
SO2 Bent 1.60
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Molecular Polarity
NH
H
H
Which has larger polarity?
NH3 NF3
NF
F
F
m =0.24 Dm = 1.46 D
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Dipole Moments
Predict whether the following
molecules are polar.
IBr
CH2Cl2AlCl3
Remember to distinguish between
bond polarity and molecular polarity.
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Both H2 & F2 have single bonds,
but…
Bond
length
Bond
energy
H2 74 pm 436 kJ
F2 142 pm 151 kJ
Shortfall of VSEPR
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Quantum Mechanics…
Valence Bond Theory
e- in molecule occupy
blended atomic orbitals
…to the rescue
Molecular Orbital Theory
molecule has
“molecular orbitals”
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Valence Bond Theory
Atomic orbitals (s, p, d …)
of the valence electrons
“hybridize” or mix to form new
orbitals for the molecule.
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Valence Bond Theory
2s 2pcarbon
atomic
orbitalsC
How can carbon form four equal
bonds with four hydrogen atoms
using its atomic ‘s’ and ‘p’ orbitals?
Consider tetrahedral CH4
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CH4: sp3 Hybridization
2s 2p
carbon
hybridized
orbitals
carbon
atomic
orbitalsC
Csp3 (all the same)
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Hybridization Analogy
s p p p
4 sp3 hybrid orbitals
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sp3 Hybridization
It takes energy to form hybrid
orbitals, but this energy is
more than compensated by
bond formation.
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sp3 Orbital Shapes
Each
orbital
can hold
2 e-
tetrahedral
One s & three p orbitals change
to four sp3 hybrid orbitals.
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CH4 Bonding
The 4 sp3 hybrid
orbitals of C bond
(overlap) with the
4 1s atomic orbitals
of the H atoms.
H
HHH
C
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NH3 sp3 Bonding
nitrogen
hybridized
orbitals
nitrogen
atomic
orbitals
2s 2pN
sp3 (all the same)
N
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sp3
bonding orbitalslone pair
NH3 sp3 Bonding
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Hybridization
First use VSEPR to predict the
arrangement of e- pairs, then
use hybridization to predict the
type of bonding.
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sp Hybridization
Be atomic
orbitals
2s 2pBe
Be hybrid
orbitals
sp 2p
Be
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sp Hybridization: BeCl2
BeCl Cl
one atomic p
orbital of Cl
two sp hybrid
orbitals of Be
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sp2 Hybridization
B atomic
orbitals
2s 2p
B
B hybrid
orbitals
sp2 2p
B
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sp2 Hybridization: BF3
B
F F
F
one p orbital
of F
three sp2
hybrid orbitals
of B
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Hybridization
Hybridization for 2nd Period
elements (C, N, O, F) explains the
“octet” rule, since there are 4 hybrid
orbitals formed from one s and three
p atomic orbitals.
s p sp3
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Hybridization: Review
1.Not applied to isolated atoms
2.First determine VSEPR geometry
3.Mix nonequivalent atomic orbitals
of central atom to form hybrid
orbitals
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Hybridization: Review
4.Requires energy, but energy is
more than returned by bond
formation
5.Covalent bonds formed by
overlap of hybrid-hybrid and/or
hybrid-unhybridized orbital
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Hybridization: Try It
Determine hybridization in:
AlBr3PF3
HgCl2
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Let’s Not Forget d Orbitals
For elements in the 3rd Period and
higher, hybridization can also
include ‘d’ orbitals.
d-orbital hybridization is still
being debated!
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SF
F
F
F
F
F
d Hybridization
SF6 VSEPR predicts
octahedral geometry
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Hybridization: SF6
S atomic orbitals
S hybridized sp3d2 orbitals
3s 3p 3d
sp3d2 3d
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Hybridization
You Try It .
What is the VSEPR geometry
and the hybridization in PBr5?
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Summary: Hybrid Orbitals
2 e- pairs (sp) linear
3 e- pairs (sp2)
trigonal planar
4 e- pairs (sp3)
tetrahedral
e- domains (bonding & nonbonding pairs)
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Summary: Hybrid Orbitals
5 e- pairs (sp3d)
trigonal bipyramid
6 e- pairs (sp3d2)
octahedral
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Multiple Bonds
In VSEPR, no distinction was made
among single, double, triple bond or lone
pair. All were counted as an “e- domain”.
Each carbon is trigonal planar
C2H4 C CH
H
H
H
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Double Bond: C2H4
2s 2pC atomic
orbitals
C hybrid
orbitals
sp2 pz
+
unhybridized
p orbital
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Double Bond: C2H4
C hybrid
orbitals
sp2 pz
+
trigonal planar dumb bell
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Double Bonds: C2H4
C C
H
HH
H
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Double Bond: C2H4
Sigma (s) and Pi (p) Bonds
(model)
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Pi & Sigma Bonds
Pi (p) bond: covalent bond formed
by sideways e- overlap above and
below the plane connecting atoms
(weaker than s)
Sigma (s) bond: covalent bond
formed by e- overlap along the
axis connecting atoms
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Double Bond: C2H4
The double bond is one sigma and one
pi bond between the carbon atoms
C CH
H
H
H
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Triple Bond: H-C=C-H
2s 2pC atomic
orbitals
C hybrid
orbitals
sp py pz
+unhybridized
p orbitals
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Triple Bond: C2H2
Hybridize s bonds only
hybridized
sp orbitals
unhybridized
p orbitals
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Hybridizing Shortcuts
1.Single bonds: sigma2.Double bonds: one sigma, one pi3.Triple bonds: one sigma, two pi4.Hybridize: add the number of
sigma bonds plus lone pairs
s p d
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70
You Try It
Describe the bonding and
hybridization for each atom in:
•formaldehyde, CH2O
•hydrogen cyanide, HCN
(assign formal charges)
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Delocalized Molecular Orbitals
An explanation of resonance.
O OOO O O
ozone
Ozone is a blend of the two
resonance structures.
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Delocalized Molecular Orbitals
72
The p component of the double bond
is delocalized over the molecule.
sp2
sp2
sp2
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Delocalized “MOs”
e- in s bonds– localized
e- in p bonds-- delocalized
H
H
H
H
H
H
H
H
H
H
H
H
benzene
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Benzene
sp2
sp2
C
C
C C
C
C
Sigma bonds
(localized)
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Benzene
p bonds
(delocalized)
sp2 pz
+
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Benzene
Bond order 1.5
H
H
H
H
H H
“Conjugated” double bonds are
more stable (less reactive)
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Carbonate Ion
+ resonance
structuresO—CO
O
2-
C: sp2 fors bonds (localized)
plus a 2pz for p bond (delocalized)
O: each has a 2pz orbital (delocalized)
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Carbonate Ion
o
o
oC
Additional stability due to
delocalization of the pi bond.o
o
oC
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79
You Try It
Would you predict the NO3- anion
to have additional stability due to
delocalization of the p electrons?
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Polyatomic Ions
Many common polyatomic ions
have delocalized pi bonds,
partially accounting for their
additional stability in chemical
reactions.
CO3-2, NO3
- , ClO3- etc.
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Ionic & Covalent
CaCO3
C
O
OOCa
2-
2+
Trigonal planar, sp2, 120o
Ionic compounds containing polyatomic
ions have both ionic and covalent bonds!