Transcript Valence Bond Theory

ORBITAL HYBRIDIZATION:
The question of shape!
We need next to examine the relationship between:
• isolated atoms (with valence e’s in s,p, and d orbitals
of specific shapes, see next slide as review!)
• bonded atoms in molecules or ions, in which
bonded regions exhibit significantly different
shapes as described by VSEPR theory
Orbital shapes, Individual (“isolated”) Atoms
all s orbitals
all p orbitals
d orbitals
Compare (next slide) to molecule, ion bonding shapes
trigonal
trigonal-bipyramidal
tetrahedral
octahedral
To rationalize how the shapes of atomic orbitals
are transformed into the orbitals occupied in
covalently bonded species, we need the help of
two bonding theories:
Valence Bond (VB) Theory, the theory we will explore,
describes the placement of electrons into bonding
orbitals located around the individual atoms from
which they originated.
Molecular Orbital (MO) Theory places all electrons from
atoms involved into molecular orbitals spread out over
the entire species. This theory works well for excited
species, and molecules like O2. You will meet this theory
in advanced classes!
COVALENT BOND FORMATION (VB THEORY)
In order for a covalent bond to form between
two atoms, overlap must occur between the orbitals
containing the valence electrons.
The best overlap occurs when two orbitals are allowed
to meet “head on” in a straight line. When this occurs,
the atomic orbitals merge to form a single bonding
orbital and a “single bond” is formed, called a
sigma () bond.
Dotted areas: representation of "electron cloud" for one electron
"Head-on Overlap"
Sigma Bond: merged orbital, 2 e's
MAXIMIZING BOND FORMATION
In order for “best overlap” to occur, valence electrons
need to be re-oriented and electron clouds reshaped
to allow optimum contact.
To form as many bonds as possible from the available
valence electrons, sometimes separation of electron
pairs must also occur.
We describe the transformation process as “orbital
hybridization” and we focus on the central atom in
the species...
“sp” Hybridization: all 2 Region
Species
BeCl2
Cl
Be
Be 2
2Cl 14
Cl
16 e's/2= 8 prs
Cl
Be
Cl
(octet violator)
Number of regions around CENTRAL ATOM: 2
Cl
Be
Cl
shape : LINEAR
bond angles: 180o
Hybridization of Be in BeCl2
Valence e’s
Atomic Be: 1s2 2s2
Hybrid sp orbitals:
1 part s, 1 part p
Energy
2p
separate
2p
"hybridize"
"sp"
2s
2s
"sp"
"arrange"
Be
Be is said to be
"sp hybridized"
(VSEPR)
FORMATION OF BeCl2:
Each Chlorine atom, 1s22s22p63s23p5 , has one
unshared electron in a p orbital. The half filled
p orbital overlaps head-on with a half full hybrid
sp orbital of the beryllium to form a sigma bond.
Be
Cl
Cl
Cl
Be
Be
Cl
Cl
Cl
sp hybridized, linear, 180o bond angles
“sp2” Hybridization: All 3 Region Species
F
BF 3
F
B
F
B 3
3F 21
24 e's/2= 12 prs
F
F
B
F
(octet violator)
Number of regions around CENTRAL ATOM: 3
F
F
shape : TRIGONAL PLANAR
bond angles: 120o
B
F
Hybridization of B in BF3
Valence e’s
Hybrid sp2 orbitals:
1 part s, 2 parts p
Atomic B : 1s2 2s2 2p1
Energy
2p
separate
2p
"hybridize"
"sp2" "sp2" "sp2"
2s
2s
"arrange"
B
B is said to be
"sp2 hybridized"
(VSEPR)
FORMATION OF BF3:
Each fluorine atom, 1s22s22p5, has one unshared
electron in a p orbital. The half filled p orbital
overlaps head-on with a half full hybrid sp2 orbital
of the boron to form a sigma bond.
sp2 hybridized, TRIGONAL PLANAR,
120o bond angles
F
F
B
B
F
F
F
F
“sp3” Hybridization: All 4 Region Species
H
CH4
H
C
H
C 4
4H 4
8
8 e's /2 = 4 pr
H
Number of regions around CENTRAL ATOM: 4
H
shape : TETRAHEDRAL
bond angles: 109.5o
C
H
H
H
Hybridization of C in CH4
Valence e’s
Hybrid sp3 orbitals:
1 part s, 3 parts p
Atomic C : 1s2 2s2 2p2
Energy
2p
separate
2p
"hybridize"
"sp3" "sp3" "sp3" "sp3"
2s
2s
"arrange"
C
C is said to be
"sp3 hybridized"
(VSEPR)
FORMATION OF CH4:
Each hydrogen atom, 1s1, has one unshared
electron in an s orbital. The half filled s orbital
overlaps head-on with a half full hybrid sp3 orbital
of the carbon to form a sigma bond.
sp3hybridized, TETRAHEDRAL,
109.5o bond angles
H
H
C
C
H
H
H
H
H
H
Unshared Pairs, Double or Triple Bonds
Unshared pairs occupy a hybridized orbital the
same as bonded pairs: See the example of NH3
that follows.
Double and triple bonds are formed from electrons
left behind and unused in p orbitals. Since all
multiple bonds are formed on top of sigma bonds,
the hybridization of the single () bonds determine
the hybridization and shape of the molecule...
N
3H
NH 3
H
N
H
5
3
8e's/2=4 prs
H
H
N
H
H
Number of regions around CENTRAL ATOM: 4
shape : TETRAHEDRAL
bond angles: < 109.5o
N
H
H
H
Hybridization of N in NH3
Valence e’s
Atomic N: 1s2 2s2 2p3
Energy
2p
"hybridize"
"sp3" "sp3" "sp3" "sp3"
2s
"arrange"
N
N is said to be
"sp3 hybridized"
(VSEPR)
FORMATION OF NH3:
Each hydrogen atom, 1s1, has one unshared
electron in an s orbital. The half filled s orbital
overlaps head-on with a half full hybrid sp3 orbital
of the nitrogen to form a sigma bond.
sp3hybridized, TETRAHEDRAL,
~107o bond angles
N
N
H
H
H
H
H
H
Describe Hybridization of C and shape of following
species:
CO, CO2, HCN, CH2O, CO32- , CBr4
C
H
O
O
C
O
O
C
C
H
O
O
H
2
C
N
Br
C
O
Br
Br
Br
“sp3d” Hybridization: All 5 Region Species
F
F
F
P
PF5
P
5
5F 35
40
40 e's /2 = 20 pr
F
F
F
F
P
F
F
F
Number of regions around CENTRAL ATOM: 5
F
F
P
F
F
F
shape : TRIGONAL BIPYRAMIDAL
bond angles: 90, 120, 180o
Hybridization of P in PF5
P: 1s2 2s2 2p6 3s2 3p3
Energy
3d
3d
separate
3p
3p
3s
3s
"hybridize"
"sp3d" "sp3d" "sp3d" "sp3d"
"sp3d"
"arrange"
(VSEPR)
P
P is said to be
"sp3d hybridized"
FORMATION OF PF5:
Each fluorine atom, 1s22s22p5, has one unshared
electron in a p orbital. The half filled p orbital
overlaps head-on with a half full hybrid sp3d orbital
of the phosphorus to form a sigma bond.
sp3d hybridized, TRIGONAL BIPYRAMIDAL,
90, 120, 180o bond angles
F
F
F
F
P
F
P
F
F
F
F
F
“sp3d2” Hybridization: All 6 Region Species
F
F
SF6
S
S
6
6F 42
48
48 e's /2 = 24 pr
F
F
F
F
F
F
S
F
F
F
F
Number of regions around CENTRAL ATOM: 6
F
F
F
S
F
F
F
shape : OCTAHEDRAL
bond angles: 90, 180o
Hybridization of S in SF6
S: 1s2 2s2 2p6 3s2 3p4
Energy
3d
3d
separate
3p
3p
3s
3s
"hybridize"
"sp3d2"
"arrange"
(VSEPR)
S
S is said to be
"sp3d2 hybridized"
FORMATION OF SF6:
Each fluorine atom, 1s22s22p5, has one unshared
electron in a p orbital. The half filled p orbital
overlaps head-on with a half full hybrid sp3d2 orbital
of the phosphorus to form a sigma bond.
sp3d hybridized, TRIGONAL BIPYRAMIDAL,
90, 120, 180o bond angles
F
F
F
F
F
S
S
F
F
F
F
F
F
F
Group Work 13.2
Describe hybridization of S and shape of species
in SF2, SO2, SO32- , SF3+, SF4, SF52S
S
S
F
O
F
O
O
O
O
F
F
S
F
F
S
S
F
F
F
F
F
F
F
F
Summary: Regions, Shapes and Hybridization
#, regions
shape
2
linear
3
4
5
6
trigonal
planar
tetrahedral
trigonal
bipyramidal
octahedral
hybridization
sp
sp
2
sp
3
3
sp d
3 2
sp d
“BOTTOM LINE”
IF you can draw a Lewis structure for a species,
and count electronic regions around central atom,
you can immediately determine:
• the shape of the species about the central atom
• the hybridization of the species based on the
central atom