Lecture on

1
Chapter 10

• Lecture on
• Molecular Geometry
• Friday January 21st 2005

2
Introduction to Bond Theory

• Lets consider chemical compounds solely in the
  atomic composition
• Lets compare BF3 and PF3
• B P
• They both seem to have the Formula AX3

F
F
F
3
Bond Angles

• Recall that bonds represent electrons (2)
• Therefore, they will try to stay as far away from
  each other as possible
• If we have 3 pairs of electrons what is the
  farthest they can be from each other?
• What is the angle of a equilateral triangle?
• 120

4
Do they have the same Shape?

• So it was thought that both had the same
  molecular shape
• Indeed the bond angles for the F-B-F set is 120
• However, the F-P-F ? 120 96
• Therefore there must be something else going on

5
Was the Lewis Structure Right?

• Lets revisit the Lewis Structure of PF3
• Phosphorus starts with 5 valence e-s
• It has shared 3 so far so we must
• still have a pair of electrons unpaired.
• P
• This pair of electrons pushes the other bonds
  down so the angle is lt120 in reality this is
  not a trigonal planar but some sort of
  tetrahedral

..
6
Isomers

• Other molecular shape differences lead to Isomers
  (which will be discussed later in the term or in
  O-Chem)
• We will discuss (cis- vs trans-)
• CC CC

Cl
H
H
H
Cl
H
Cl
Cl
trans- means across the
cis- means on the same side of the
7
Why is a double bond Important?

• If all we have is single bonds, the atoms are
  free to rotate.
• The addition of a second bond to the same atom
  restricts rotation so while in a single bond we
  can rotate a trans back to a cis we cannot do
  that with a

8
So how do I know PF3 is a type of tetrahedral?

• Our theory now is that the extra pair of
  electrons in the P has an effect on the shape.
• This is the essence of Valance Shell Electron
  Pair Repulsion Theory.
• VSEPR Theory not only considers the number of
  atoms bonded to the central atom but how many
  lone pair of electrons are there as well.
• Recall once again that bonds represent electrons
  and they want to be as far from each other as
  possible.

9
Electron Arrangement

• Lets work with electrons and no atoms for now.
• If we only have one pair of electron for bonding
  we will obviously make a linear molecule so we
  wont bother with these.
• If we have 2 pairs we will make a linear molecule

10
Linear to Trigonal Planar
180
As a 3rd pair of electrons are introduced The 2
pairs present will start feeling Coulombic force
and be repelled down
120
11
Trigonal Planar to Tetrahedral
Molecules take advantage of the 3D of our world
and the new pair of electrons comes in at the top
as opposed to the side. As the electrons are
brought in The 3 existing pairs are pushed down
forming a tripod type of arrangement. The final
shape has 4 identical sides this figure is
called a tetrahedral.
109.5
12
Tetrahedral to Trigonal Bipyramidal
As the 5th pair of electrons is added the 3 sets
that had formed the tripod are now repealed
upwards.
90
The repulsion from the top pair of electrons has
the same magnitude as the force from the new pair
of electrons so 3 pairs that were the legs of the
tripod end up in a horizontal plane at the
equator.
120
180
13
Trigonal Bipyramidal to Octahedral
As the 6th and last pair of electrons approach
the best place is along the equator between two
of the existing pairs of electrons this starts
pushing those 2 pairs away from a trigonal
arrangement to a square.
90
The resulting shape is what may be described as a
Square Bipyramidal or since the figure has 8
identical sides an octahedral.
180
14
Molecular ShapeLinear Electron Arrangement

• Now lets start bringing in Atoms and determine
  shape.
• In the case of a linear Electron Arrangement we
  have 2 choices
• 2 bonded atoms to the central atoms, which
  results in a linear shape
• This type of molecules have a Class AX2 (A is
  the central atom, and it has 2 bonded atoms (X)
  to itself.
• 1 bonded atom and 1 pair of unshared electrons,
  but we said we will not deal with these type of
  compounds.

15
Molecular ShapeTriangular Planar Electron
Arrangement