Five Basic Geometries

1
Five Basic Geometries
Linear
Trigonal
Tetrahedral
Octahedral
Trigonal bipyramidal
2
SeF6, IF5, and XeF4
3
SeF6 Octahedron
All bond angles are 90
4
IF5 and XeF4

• The 1st lone pair can occupy any site
• The 2nd lone pair is arranged opposite to the 1st

IF5
XeF4
Square Pyramidal
Square Planar
5
Octahedral Electronic Geometry

• If lone pairs are incorporated into the
  octahedral structure, there are 2 possible new
  shapes
• 1 lone pair Square pyramid
• 2 lone pairs Square planar

6
Chemical Bonding

• How are the chemical bonds formed?
• Formation of a chemical bond usually involves
  pairing of unpaired electrons from the atoms
  being bound
• Example H2 molecule
• H H ? HH

7
BeH2

• The Be atom has 2 paired electrons
• How does it form 2 equivalent bonds?
• To answer questions like this, the valence bond
  theory was proposed

8
Valence Bond Theory

• When an atom is nearby other atoms, its outer
  shell orbitals can mix and get modified
• They form a new set of orbitals that are more
  appropriate for bonding
• This process is called hybridization
• The new orbitals are therefore called hybrid
  orbitals
• Hybrid orbitals are arranged in the same way as
  predicted by VSEPR

9
BeH2
Be 1s22s2
sp - hybridization
10
BeH2
sp
sp
1s
1s
Be


H
H
11
BF3
12
BF3
B 1s22s22p1
sp 2 - hybridization
13
BF3
B

3
F
2p
14
CH4 and CF4
109.5
15
CH4 and CF4
C 1s22s22p2
sp 3 - hybridization
16
CH4

4
H
1s
17
CF4

4
F
2p
18
NH3 and NF3
NH3
NF3
107.3
102.1

• Just like in CH4 and CF4, the orbitals are
  arranged in the tetrahedral fashion which means
  that the sp 3 hybridization takes place
• One of the orbitals, however, contains a pair of
  electrons and is not used for bonding

19
NH3 and NF3
N 1s22s22p3
sp 3 - hybridization
20
NH3 and NF3
N

3
H
1s
21
NH4
1s

H

22
PF5
P Ne3s23p3
sp 3d - hybridization
23
PF5
Trigonal bipyramidal electronic geometry is
achieved by sp 3d - hybridization
24
SF6
S Ne3s23p4
sp 3d 2 - hybridization
25
SF6
Octahedral electronic geometry is achieved by sp
3d 2 - hybridization
26
VB vs. VSEPR Theories
Regions of High Electron Density Electronic Geometry Hybridization
2 Linear sp
3 Trigonal planar sp2
4 Tetrahedral sp3
5 Trigonal bipyramidal sp3d
6 Octahedral sp3d2
27
Double Bond C2H4

• An sp 2 hybridized C atom has one electron in
  each of the three sp 2 lobes

Side view of the sp 2 hybrid the unhybridized
p orbital
Top view of the sp 2 hybrid
28
Double Bond C2H4

• Two sp 2 hybridized C atoms plus p -orbitals in
  proper orientation to form a CC double bond

29
Double Bond C2H4

• The portion of the double bond formed from the
  head-on overlap of the sp 2 hybrids is designated
  as a s bond
• The other portion of the double bond, resulting
  from the side-on overlap of the p orbitals, is
  designated as a p bond

30
Triple Bond C2H2

• A ? bond results from the head-on overlap of two
  sp hybrid orbitals

31
Triple Bond C2H2

• The unhybridized p orbitals form two p bonds

• Note that a triple bond consists of one ? and two
  p bonds

32
Assignments Reminders

• Go through the lecture notes
• Read Chapter 8 completely
• Homework 5 covers Chapters 7 8 and is due by
  Oct. 31
• Monday (10/31) and Tuesday (11/1) lecture quiz
  5 (Chapter 8)