Download 6-5 Notes VSEPR

6­5 Notes VSEPR
January 06, 2012
You only need to
write the words in
black!
Chapter 6 ­ Section 5
Molecular Geometry
VSEPR Theory ("vesper")
Stands for valence shell, electron­pair repulsion theory
It states: repulsions between the sets of valence­level electrons surrounding an atom causes these sets to be oriented as far apart as possible.
In other words: valence shell electrons will move so that they are the farthest away from each other as possible.
How does this account for molecular shape?
1
6­5 Notes VSEPR
January 06, 2012
It is first easier to look at molecules with NO LONE PAIRS on the CENTRAL atom.
EX: BeF2
First, draw the Lewis Structure:
Accoring to VSEPR theory, the shared pairs will be as far apart as possible. Think of a sphere or ball around the central Be. There are two F's attached to it. A sphere is 360 degrees. If we divide the 360 by 2 (360/2) we will get 180 degrees. The term for this geometry is linear. (as in a straight line) The bond angle is 180 degrees.
2
6­5 Notes VSEPR
January 06, 2012
Notice that linear geometry can occur with molecules that have a central atom (with NO lone­pairs) attached to two attritional atoms. (As in the previous example of BeF2.)
Linear geometry is ALWAYS the case when there are only two atoms bonded together like the below examples:
What would the bond angle be?
3
6­5 Notes VSEPR
January 06, 2012
Now, think about what a central atom attached to 3 elements would look like. (It helps to think about dividing a circle again.)
Use BF3 as an example.
First, draw the Lewis Structure: Then think about how the F's would need to move to be the farthest apart from one another.
The molecule would be flat with the three outer F's looking like a triangle. The term for this geometry is trigonal­
planar. The bond angles are all 120 degrees. 4
6­5 Notes VSEPR
January 06, 2012
Until this next example, the molecular geometry has been in one "plane" of space. We are now going to look at "3D" molecules that exist in more than one plane.
EX: CH4
Draw the Lewis Structure: Now, think again about the 360 degree sphere. By what number should we divide 360 to determine how far apart the atoms are?
Four. 360/4 = 90 degrees. We would think that all of the atoms should be 90 degrees apart, but that would be on one plane. The atoms can actually get a little farther apart...
5
6­5 Notes VSEPR
January 06, 2012
In reality, the 4 hydrogen atoms (outer atoms) spread to be 109.5 degrees apart. they take the shape of a tetrahedron (a 4 sided shape)
The molecular geometry is tetrahedral. The bond angles are 109.5 degrees.
6
6­5 Notes VSEPR
January 06, 2012
For an atom attached to FIVE others atoms, the molecular geometry is trigonal­bipyramidal. The bond angles are 120 degrees (around the "center") and 90 degrees (from middle to top and middle to bottom).
EX: PF5
Again... draw the Lewis Structure:
When put into "3D" it will look like:
The Blue angles
are 120 degees
(like trigonalplanar)
The green angles
are 90 degrees
90 degrees
120 degrees
7
6­5 Notes VSEPR
January 06, 2012
For an atom attached to SIX others atoms, the molecular geometry is Octahedral. The bond angles are ALL 90 degrees (from one atom to a neighboring atom).
EX: SCl6
Again... draw the Lewis Structure:
When put into "3D" it will look like:
You can see the "4" around the center and one on the top and one on the bottom
8
6­5 Notes VSEPR
January 06, 2012
Let's PRACTICE!!!
Please fill in the following chart while you build your molecules with the sets.
Molecular
Geometry
1
BI3 2
HF 3 SF6 4 PBr5 5 SiH4
6 CH2O
L.P. of
# atoms electrons
bonded to
central atom
Lewis
Structure
9
6­5 Notes VSEPR
January 06, 2012
Chart from page 200 of your text
10
6­5 Notes VSEPR
January 06, 2012
11
6­5 Notes VSEPR
January 06, 2012
It helps to look at the electron­pair geometry FIRST
12