Molecular Shape
Drawing Good Lewis Structures
1. # valence
Always.
e–
in atoms (± charge) must = #
e–
Ronald J. Gillespie
English
in structure.
1957
2. Determine connectivity: least EN usually central, avoid small
rings, H always terminal (1 e–).
3. Complete octet for each atom (except H); check against #1.
Valence-Shell Electron-Pair Repulsion
VSEPR
regions of e– ρ around central atom repelled as far as possible
••
4. Remove required e– in pairs from central atom.
5. Move e– pairs from outside atoms to bond with central atom
to complete octet again.
x
6. Minimize formal charge (# and distribution of + and –).
=
3
linear
x
x
trigonal
bipyramidal
octahedral
x
trigonal planar tetrahedral
H2 Molecule
109.5o
120o
109.5o
bent
trigonal planar
0
trigonal pyramidal tetrahedral
Energy (eV)
120o
bent
=
4
180o
linear
=
x
VSEPR and Deviations
2
Ronald S. Nyholm
Australian
6
5
180o
90o
90o
90o
90o
90o
120o
linear T-Shaped see-saw trigonal bipyramidal square planar square pyamidal octahedral
1
–2
2
3
r (Å)
bonding-bonding < Lp-bonding < Lp-Lp
Overlap Symmetry
cross
section
1.0
0.5
deviate from ideal when lone-pair involved

experiment
i
t
–4
Valence Bond Treatment: CH4

½(s + px + py + pz)
cross
section
½(s + px – py – pz)
½(s – px + py – pz)

4 sp3 orbitals
H
½(s – px – py + pz)


C
s


S σ > Sπ > Sδ
p
H

2p
E

2s
d
H
H
sp3
weighted average E
1
sp2 Hybrid Orbitals
1
√3
1
√3
1
√2
s +
px
√3
√3
1
1
px +
s –
py
√6
√2
1
1
px –
s –
py
√6
√2
sp Hybrid Orbitals
1
(s + pz)
√2
1
(s – pz)
√2
sp2
2p
2s
s
s
s
s
s
s
dsp3
+
–
+
+
–
–
px
px
pz
py
px
py
+
–
–
–
–
–
sp
2s
Other Hybrid Orbitals
dz2
dz2
dz2
dz2
dz2
dz2
2p
2p
E
sp2
equatorial s + px
s – px + py
s – px – py
axial pz + dz2
pz – dz2
linear
trigonal planar
2p
E
sp
Hybrid Orbitals and Bond Strength
bond strength  S
–
–
+
–
d x 2 – y2
d x 2 – y2
d x 2 – y2
d x 2 – y2
d2sp3
s character  S
sp > sp2 > sp3
SC-C
0.8
SC-H
0.7
S 0.6
0.5
0.4
sp3 sp2 sp
50% s
trigonal
bipyramidal
0.9
33% s
25% s
0.3
0
20
40
60
80 100
% s character
octahedral
Multiple Bonds
Non-VSEPR Molecule
Multiple bonds from π (and δ) overlap.
N(SiH3)3
••
••
O
ClO3–
– ••
••
O Cl O
•• •• ••
Cl has low E d orbitals
O sp2
sp2
O
D3h not C3v
Cl
O sp2
Si
Si
Si
N
Si
Si
Si
sp2 N
low E d on Si
more bonds, lower E
sp3
2
MO Treatment H2
MO Treatment H2
no e– density between nuclei – antibonding (u)
Bond Order = (# of bonding e– – # of antibonding e–)/2
1s
1s
1s
Energ
gy
Energ
gy
E
1s
1s
1s
E
1s
E = E
H2 lower energy
than 2 H by 2 x E.
1s
Bond Order = (2 – 0)/2 = 1
e– density between nuclei – bonding (g)
MO Treatment He2
Bond Order = (# of bonding
e–
Molecular Orbitals
– # of antibonding
S depends on E and symmetry: SAB > 0, bonding:
E stabilized
SAB < 0, antibonding: E destabilized
Sσ > Sπ > Sδ
SAB = 0, nonbonding: no stabilization
e–)/2
Energ
gy
1s
1s
1s
–
–
π*2p g
+
+
π2p
–
*2p u
+
2p
Bond Order = (2 – 2)/2 = 0
–
*1s u
No energy advantage: He2 does not exist.
+
1s
1s
s-p Energy Separation in First Row Elements
B
C
2p
Energy
Complicated by s-p mixing
when s and p close in E.
Changes relative MO E’s.
N
2p
Energy
C
N
O
O
2p
2s
2p
*2p
2p
2s
2s
8.8
12.4
16.5
2px
2py
2pz
Complicated by s-p mixing
when s and p are closer in
E (early elements).
2s
5.7
g
*2p
F
F
E (eV)
g
Homonuclear Diatomic MO Diagram
2s
*2p
2p
*2s
2s
B
u
21.6
2p
2pz
2py
2px
2p
*2s
2s
2s
2s
3
Homonuclear Diatomic MO Diagram
2px
2py
2pz
No longer named after
AO. Numbered and
symmetry (u or g) given.
MO Diagram: Li2, Be2, B2, C2, N2
6u
6u
2g
2g
5g
2pz
2py
2px
2px
2py
1u
2s
2s
Li
MO Diagram: Li2, Be2, B2, C2, N2
2pz
B 2
Be
6u
2g
2g
2pz
2py
2px
2px
2py
2pz
B2
Be
2s
Be
B
MO Diagram: Li2, Be2, B2, C2, N2
2px
2py
2pz
C2
2g
2g
2pz
2py
2px
2px
2py
2pz
N2
C
B
5g
2pz
2py
2px
4u
2s
3g
2px
1u
bond order
4u
2s
2py
MO Diagram: Li2, Be2, B2, C2, N2
6u
5g
2pz
2s
3g
6u
1u
bond order
5g
4u
2s
3g
Li
1u
bond order
4u
2s
2px
MO Diagram: Li2, Be2, B2, C2, N2
1u
bond order
2py
2s
3g
6u
5g
2pz
4u
3g
2py
5g
1u
bond order
4u
2s
2px
2pz
Li2
C
2s
N
2s
3g
N
4
MO Diagram: O2, F2, Ne2
2px
2py
2pz
*2p
*2p
*2p
*2p
2p
O2
MO Diagram: O2, F2, Ne2
2pz
2py
2px
2px
2pz
2pz
*2s
2s
2s
2s
2s
O
2s O
F
2s F
Bond Length  1/Bond Order
MO Diagram: O2, F2, Ne2
2py
*2p
*2p
*2p
*2p
2pz
2pz
N 2
Ne
bond order
2py
2p
2p
2p
*2s
*2s
2s
O
2s Ne
Cr2:  Bonds
2s
order length, pm
O2+
O2
O2–
O22–
O
Carbon Monoxide MO Diagram

*s
*z2
Cr2
bond order
Superoxide
dismutase
(SOD)
2px
2p
2s
Ne

2p
*xz, yz
*x2–y2, xy
2p

x2–yy2 xyy
yyz
3d
xz
z2
4s
Cr
z2
x2–y2, xy
xz, yz
 z2
2px
2p
bond order
*2s
2px
2py
2p
F2
2p
bond order
2py
xz
yyz
3d
xyy x2–yy2
nb

2s
4s
Cr
xz, yz
 z2
s
x2–y2, xy
C
nb
2s
O
5
H2 O
CH4 SALC
O
H
H
b1*
A1 = H1 1s + H2 1s + H3 1s + H4 1s
O
H
a1*
H
T = H1 1s – H2 1s + H3 1s – H4 1s
T = H1 1s – H2 1s – H3 1s + H4 1s
B1
A1
2pp
A1
B1
B2
H
H
H
O
b1
H
H
O
1a1
A1
H
O
2a1
2s
T = H1 1s + H2 1s – H3 1s – H4 1s
O
b2
H
O
H
A1
bonding orbitals
2H
Diborane, B2H6: 3-Center, 2 e– Bond
B
H
Electronegativity: Periodic Property
H B SALC: A1g = sp3 + sp3
H
H
H
D2h
B2u = sp3 – sp3
B
4.5
H: A1g = 1s
H
F
4.0
Electronegativity
y
3.5

nb
B2u
A1g
A1g
B, B
Cl
30
3.0
At
1.5
Li Na
K
1.0
Total Energy
y (MJ/mole)
Rb
0
20
10
Tottal Energy
6
2
3
N
Na
4
where
q = ionic charge
Cl larger  than Na
8
2
E = IE or EA
20
Cl
12
E = q + q2
45
100
14
Cl
70
80
Mulliken-Jaffe Electronegativity
95
1
Fr
40
60
Atomic Number
Ne
F
O
Oxidation State
Cs
0.0
120
0
I
2.0
Total Energy
–5
Br
2.5
0.5
H

–1
T
(~98% O)
0
–1
1
–2
–4
Charge
6
Electronegativity Equalization
14
Cl E = 11.0q + 5.7q2
12
Tottal Energy
10
Na0.51+Cl0.51–
8
6
N E = 2.8q
Na
2 8 + 2.3q
23 2
4
a A  aB
= δ
bA + bB
2
0
–1
–2
1
11.0  2.8
= 0.51
2(5.7) + 2(2.3)
–4
Charge
7