Chem 14A

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Chem 14A

• Wednesday, July 11, 2007

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Lecture Outline

• VSEPR model for molecular structure
• Molecular Geometry
• Electronic Geometry
• Orbital Hybridization
• s and p bonds

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VSEPR

• How do we determine the structure of molecules?
• Valence Shell Electron Pair Repulsion Theory
• Basic Principle In order to limit electrostatic
  repulsion, electron pairs in atomic orbitals stay
  as far apart as possible

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VSEPR

• How do we use VSEPR theory to determine the
  structure of molecules?
• When using VSEPR, it is important to remember the
  following
• electrons repel each other
• valence electron pairs stay as far apart as
  possible
• non-bonding electrons repel more than bonding
  electrons

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PREDICTING THE GEOMETRY OF MOLECULES
Step 1. Draw Lewis structure of the molecule
Step 2. Count the bonding and non-bonding
electron pairs
Step 3. Using the total number of bonds and lone
pairs, find the molecular structure from the
vsepr chart
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Basic Molecular Shapes
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AB2
BeCl2
Be
Cl
Cl
TWO ELECTRON PAIRS AROUND BERYLLIUM ATOM
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LINEAR ARRANGEMENT BEST
IT PUTS ELECTRON PAIRS FURTHEST APART
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AB3
BF3
F
F
B
F
THREE ELECTRON PAIRS AROUND THE BORON ATOM
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THREE ELECTRON PAIRS AROUND THE BORON ATOM
TRIGONAL PLANAR ARRANGEMENT BEST
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AB4
CH4
four electron pairs
expect square planar
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better arrangement for four electron pairs
TETRAHEDRAL
109.5
bigger than 90 in square planar
tetrahedral
4 electron pairs
put on the H-atoms
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TETRAHEDRAL
H
109.5
H
H
H
shape of CH4 is tetrahedral
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AB5
PF5
FIVE ELECTRON PAIRS AROUND PHOSPHORUS
5 electron pairs
trigonal bipyramidal
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Bond angle
F
900
F
P
F
P
F
1200
F
shape of PF5 is trigonal bipyramidal
two sets of different F atoms
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AXIAL
Bond angle
F
900
F
F
P
F
1200
EQUATORIAL
F
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AB6
SF6
six electron pairs around the sulfur atom
F
F
F
S
S
F
F
F
octahedral
6 electron pairs
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900
F
F
F
S
S
F
F
F
shape of SF6 is octahedral
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Electronic Geometry

• When there are lone pairs on a molecule, we need
  to account for the electronic geometry as a
  subset of the molecular geometry
• Electronic geometry takes into account the
  position of lone pairs in the molecule
• For example, lets look at SeO2

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AB2E
AB3
SeO2
O
Se
O
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VSEPR treats double bonds like a single bond
THREE ELECTRON PAIRS AROUND SELENIUM
ELECTRONIC GEOMETRY
TRIGONAL PLANAR
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ADD OXYGENS
SeO2 IS V-SHAPED (OR BENT)
THE MOLECULAR SHAPE IS THE POSITION OF THE ATOMS
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AB3E
AB4
H
N
H
NH3
H
electron pairs around the nitrogen atom
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H
N
H
NH3
H
PUT ON THE 3 H ATOMS
N
H
H
H
NH3 is trigonal pyramidal
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AB2E2
AB4
four electron pairs around the oxygen atom
H
O
H
PUT ON THE 2 H-ATOMS
O
O
H
H
shape of H2O is V-shaped or bent
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AB4E
AB5
SF4
F
F
F
S
F
TRIGONAL BIPYRAMID
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WHERE DOES LONE PAIR GO?
OR
lone pairs occupy the trigonal plane (the
equator) to minimize the number of 90
repulsions
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AB4E
AB3E2
AB2E3
SF4 1 lone pair See-saw shaped
ClF3 2 lone pairs T-shaped
XeF2 3 lone pairs Linear
F
F
F
F
Xe
S
Cl
F
F
F
F
F
F
lone pairs occupy the trigonal plane (the
equator) first to minimize the number of 90
repulsions
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AB5E
AB6
BrF5
Square pyramidal
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AB6
AB4E2
XeF4
lone pairs MUST BE AT 1800
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Summary of Molecular Shapes
Total valence electron pairs
Electron Pair Geometry
Lone electron pairs
Shape of Molecule
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Linear
0
Linear
0
Trigonal planar
Trigonal planar
3
1
V-shaped
0
Tetrahedral
4
Tetrahedral
1
Trigonal pyramid
2
V-shaped
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Total valence electron pairs
Electron Pair Geometry
Lone electron pairs
Shape of Molecule
0
Trig. bipyramid.
1
See-saw
Trigonal bipyramidal
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2
T-shaped
3
Linear
0
Octahedral
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Octahedral
1
Square pyramid
2
Square planar
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POLYATOMICS
molecules with no single central atom
we apply our VSEPR rules to each atom in the chain
Example ETHANOL
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ETHANOL
C2H5OH
The atoms around the carbons form a
tetrahedral arrangement
The atoms around the oxygen form a
V-shaped structure
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1. Lewis structures
2. VSEPR model
WHY DO MOLECULES FORM?
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simplest molecule H2
two H-atoms
1s1
two H-atoms approach each other and the electron
waves interact
OVERLAP to form a region of increased electron
density between the atoms
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chemical bond with electron density in between
the nuclei is called
? bond
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VALENCE BOND THEORY
a covalent bond is formed by an overlap of two
valence atomic orbitals that share an electron
pair
the better the overlap the stronger the bond
the orbitals need to point along the bonds
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VALENCE BOND MODEL
Hybrid orbital model
Step 1 Draw the Lewis structure(s)
Step 2 Determine the geometry of the electron
pairs around each atom using VSEPR
Step 3 Specify the hybrid orbitals needed
to accommodate the electron pairs on each atom
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CH4
What orbitals are used?
hydrogen atoms bond using their 1s orbitals
carbon needs four orbitals to bond with.
He 2s22p2
2s, 2px , 2py, 2pz
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1. The electronic configuration of carbon is
He 2s22p2
the orbital diagram is
He
the Lewis dot structure is
necessary to promote one 2s electron
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PROMOTE AN ELECTRON
He
He
He 2s22p2
He 2s12p3
excited state (valence state)
Lewis dot structure
four unpaired electrons
we can use these to form chemical bonds
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we need four orbitals pointing to the vertices of
a tetrahedron
orbitals are just mathematical functions
we can combine them
HYBRIDIZATION
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COMBINING ORBITALS TO FORM HYBRIDS
HYBRIDIZATION
number of atomic orbitals that are combined
IS EQUAL TO
the number of resulting hybrid orbitals
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METHANE CH4
four hybrid orbitals needed to form four bonds
s px py pz
4 sp3 hybrids
an atom with sp3 hybrid orbitals is said to be
sp3 hybridized
The four sp3 hybrid orbitals form a tetrahedral
arrangement.
EPG of 4 pairs
sp3 hybridization
What happens to the energies of the orbitals?
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When orbitals are hybridized they have the same
energy
HYBRIDIZE
2p
sp3
E
E
2s
Orbitals in free C atom
Hybridized orbitals of C atom in methane
The FOUR sp3 hybrids are DEGENERATE.

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Combine one s and three p orbitals..
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sp3 HYBRIDS
sp3 orbitals
Now form the bonds to the H-atoms...
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Each bond in methane results from the overlap of
a hydrogen 1s orbital and a carbon sp3 orbital.
Hydrogen 1s orbital
Carbon sp3 orbitals
H
Form a chemical bond by sharing a pair of
electrons.
H
H
H
Each hybrid ready to overlap with H 1s orbitals
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HYBRIDIZATION
Combine one s and one p
sp- hybrid

ADD the orbitals

?2s ?2p
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HYBRIDIZATION
Combine one s and one p
sp- hybrid
s p
?2s ?2p
What do we get?

The positive part cancels negative part
DESTRUCTIVE INTERFERENCE
The positive part adds to positive part
CONSTRUCTIVE INTERFERENCE
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HYBRIDIZATION
Combine one s and one p to give
a sp- hybrid
?2s ?2p
s p
Where is the nucleus?

REMEMBER IF WE MIX TWO WE MUST GET TWO BACK
The other combination is s - p
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HYBRIDIZATION
Combine one s and one p
a sp- hybrid


SUBTRACT the orbitals
?2s- ?2p
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HYBRIDIZATION
Combine one s and one p
sp- hybrid
SUBTRACTING THE p ORBITAL CHANGES ITS PHASE


SUBTRACT the orbitals
?2s- ?2p
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HYBRIDIZATION
Combine one s and one p
sp- hybrid
SUBTRACTING THE p ORBITAL CHANGES ITS PHASE


SUBTRACT the orbitals
?2s- ?2p
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HYBRIDIZATION
Combine one s and one p
a sp- hybrid
s - p
?2s- ?2p
What do we get?

CONSTRUCTIVE INTERFERENCE
DESTRUCTIVE INTERFERENCE
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HYBRIDIZATION
Combine one s and one p
sp- hybrid
s - p
?2s- ?2p
Where is the nucleus?

The positive part cancels negative part
We get two equivalent sp orbitals
ORIENTED AT 1800
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sp-HYBRIDIZATION
s and p orbitals
two sp-hybrids
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COMBINE one s-orbital and two p-orbitals
Get three sp2 - orbitals oriented at 1200
s and p orbitals
three sp2-hybrids
directed at 1200
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COMBINE one s-orbital and three p-orbitals
three sp3- orbitals oriented at 109.50
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MOLECULES USING sp3 HYBRIDS
all molecules that have a 4 pair EPG.
NH3, H2O, NH4 , CCl4
AMMONIA..
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AMMONIA NH3
VSEPR
N
Valence shell has four pairs
EPG is TETRAHEDRAL
Need sp3 hybrids
Nitrogen electronic configuration
HYBRIDIZE
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When orbitals are hybridized they have the same
energy
HYBRIDIZE
2p
sp3
E
E
2s
Orbitals in free N atom
Hybridized orbitals of N atom in ammonia
The FOUR sp3 hybrids are DEGENERATE.

sp3 hybridization.
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sp3 hybrids on N in AMMONIA
2p
2s
Now form a bond
Overlap H 1s..
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AMMONIA
2p
2s
Three ? bonds
One lone pair in an sp3 hybrid
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AMMONIUM ION NH4
H
2p
2s
four ? bonds.
ISOELECTRONIC WITH ?
CH4
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WATER
FOUR PAIRS
2p
2s
EPG?
TETRAHEDRAL!!
sp3
HYBRIDIZATION?
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WATER.
Overlap of two of oxygen sp3 hybrids with ..
Lone pairs in two of the sp3 hybrids.
H atom 1s orbitals.
To form two ? bonds.
Think about H3O !!!
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HYDRONIUM ION.
Overlap of Oxygen sp3 hybrids containing a lone
pair
ISOELECTRONIC WITH?
NH3
H ion empty 1s orbitals.
H
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VALENCE BOND THEORY FOR OTHER ELECTRON PAIR
GEOMETRIES
A four electron pair EPG uses sp3 hybrids
The three electron pair EPG uses sp2 hybrids
The two electron pair EPG uses sp hybrids
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EPGs
4
2
3
HYBRIDS
sp3
sp
sp2
lets look at a molecule that needs sp2
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Ethylene C2H4
trigonal planar EPG around each C-atom.
VSEPR
a HCH angle of 1200.
three hybrid orbitals on each carbon for the
trigonal planar EPG.
3 sp2 hybrids
s px py
The CARBON is sp2 hybridized
The 3 sp2 hybrid orbitals form a trigonal planar
arrangement.
sp2 hybridization
3 effective electron pairs
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FORMATION OF sp2 hybrids
VALENCE STATE C atom
GROUND STATE C atom
HYBRIDIZE
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FORMATION OF sp2 hybrids
VALENCE STATE C atom
GROUND STATE C atom
HYBRIDIZE
sp2 hybridized orbitals of C
This leaves one p orbital unhybrized.
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An sp2 hydridized C atom
sp2 - hybrid orbital
UNHYBRIDIZED p- orbital
The unhybridized p orbital is perpendicular to
sp2 plane.
Lets put it all together.
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DRAW TWO C-ATOMS
C
C
Now put the orbitals on...
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BONDING IN ETHYLENE
z
z
y
y
C
C
x
x
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BONDING IN ETHYLENE
s bond
z
z
y
y
C
C
x
x
OVERLAP the sp2 hybrids from the two carbons to
form a sigma bond between them.
PUT THE ELECTRONS IN AND..
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overlap two sp2 hybrids on each carbon with
hydrogen 1s orbitals to form sigma bonds and...
z
z
H
H
y
y
H
H
x
x
The two unhybridized p orbitals are left over to
form..
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The second part of the carbon-carbon double bond !
z
z
H
H
y
y
H
H
x
x
The two unhybridized p orbitals are left over to
form a ..
pi bond (p bond)
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The second part of the carbon-carbon double bond !
pi bond (p bond)
z
z
H
H
y
y
H
H
x
x
Electrons are shared between the unhybridized p
orbitals in an area above and below the line
between nuclei.
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THE COMPLETE PICTURE!!!!!!!
pi bond (p bond)
z
z
H
H
y
y
H
H
x
x
SUMMARY...
sp2
sigma bonds (s bond)
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pC(2p)-C(2p)
sC(sp2)-C(sp2)
s H(1s)-C(sp2)
s H(1s)-C(sp2)
s bonding
Now look at p bond
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pC(2p)-C(2p)
sC(sp2)-C(sp2)
s H(1s)-C(sp2)
s H(1s)-C(sp2)
p bonding
Now look at acetylene
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BONDING SCHEME IN ACETYLENE
pC(2p)-C(2p) TWO OF THESE!!
s H(1s)-C(sp)
s H(1s)-C(sp)
sC(sp)-C(sp)
What does this look like????
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DRAW TWO C-ATOMS
C
C
Now put the sp-orbitals on...
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OVERLAP the sp hybrids from the two carbons to
form a sigma bond between them.
z
z
y
y
C
C
x
x
Put in the unhybridized p orbitals
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OVERLAP the sp hybrids from the two carbons to
form a sigma bond between them.
z
z
y
y
C
C
x
x
OVERLAP the hydrogen 1s orbitals
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OVERLAP the C sp hybrids with H 1s to form sigma
bonds
z
z
y
y
H
H
C
C
x
x
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OVERLAP the sp hybrids from the two carbons to
form a sigma bond between them.
z
z
y
y
H
H
C
C
x
x
pi bonding?
sigma framework of s bonds
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LATERAL OVERLAP of p orbitals to form pi bonds.
z
z
y
y
H
H
C
C
x
x
two pi bonds (p bonds)
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LATERAL OVERLAP of p orbitals to form pi bonds.
z
z
y
y
H
H
C
C
x
x
SO..
two pi bonds (p bonds)
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What about molecules with more than an octet
around the central atom?
Examples PCl5, or SF4 or SiF62-
Four orbitals
Four pairs needs
Five orbitals
Five pairs needs
six pairs needs
six orbitals
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PCl5
We ignore the chlorine atoms and just describe
central atom
Need five hybrid orbitals on the phosphorus
to fit the trigonal bipyramidal EPG.
d s px py pz
5 dsp3 hybrids
5 effective electron pairs
dsp3 hybridization
Five equivalent orbitals..
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dsp3 - hybrid orbitals
TRIGONAL BIPYRAMID
EPG 5 PAIRS
900
1200
SIX PAIRS..
overlap with orbitals on chlorine to form 5 s
bonds.
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We ignore the chlorine atoms and just describe
central atom
SF6
We need six hybrid orbitals on the sulfur to
allow for the octahedral EPG and six bonds.
6 d2sp3 hybrids
d d s px py pz
d2sp3 hybridization
6 effective electron pairs
SIX equivalent orbitals..
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d2sp3 - hybrid orbitals
900
900
EXAMPLE
overlap with orbitals on fluorine to form 6 s
bonds.
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EXAMPLES
Describe the molecular structure and bonding in
XeF2 and XeF4
EPG 5 pairs
EPG 6 pairs
Linear
Square planar
dsp3 hybrids
d2sp3 hybrids
Two axial s bonds at 1800
four s bonds at 900 in a plane
Three lone pairs in equatorial hybrids
Two lone pairs in axial hybrids