Assignment:
1. Summarize in your own words the rules for writing Lewis electron-dot structures. Use and example of a simple molecule (not SO2, CO2, or H2O) to illustrate the application of these rules. NO3-
. . : O : Is that only valence electrons are included in the lewis structure.
|
N
// \
:O: :O:
``
2. For the molecule you used in example in Question1 above, explain the reasoning in how the VSEPR theory would be used to deduce the geometric shape of this molecule.
Procedure:
For each molecule in the table below, list the number of total valence electrons in the molecule, draw the Lewis structure, indicate the molecular geometric shape and the approximate bond angles in the molecule. Look each shape up on one of the following websites:
|
Molecule
|
Valence Electrons
|
Lewis Structure
|
Geometric Shape
|
Approximate Bond Angles
|
Sketch
|
|
CF4
|
4+4(7)=32
|
..
: F :
.. | ..
: F - C -- F :
'' | ''
: F :
''
|
Tetrahedral
|
|
|
|
 NH3
|
5+3(1)=8
|
..
H - N - H
|
H
|
Trigonal Pyramid
|
|
|
|
SO42-
|
6+4(6)+2=32
|
__ __2-
| .. |
| :O: |
| .. | .. |
| : O - S - O : |
| '' | '' |
| :O: |
|__ '' __|
|
Tetrahedral
|
|
|
|
NH4+
|
5+4(1)-1=8
|
_ _
| | +
| H |
| | |
| H - N - H |
| | |
| H |
|_ _|
|
Tetrahedral
|
|
|
|
H2S
|
2(1)+6=8
|
..
H - S - H
''
|
Linear
|
|
|
|
PO43-
|
|
|
|
|
|
|
HCl
|
1+7=8
|
|
|
|
|
|
NF3
|
5+3(7)=26
|
. .
N
/ | \
:F: :F: :F:
'' '' ''
|
Tetrahedral
Trigonal pyramid
|
|
|
|
SiH4
|
4+4(1)=8
|
..
:H:
.. | ..
:H -Si--H:
'' | ''
:H:
''
|
Tetrahedral
|
|
|
|
BF3
|
3+3(7)=24
|
. .
B
/ | \
:F: :F: :F:
'' '' ''
|
Tetrahedral
|
|
|
|
C2H6
see note
|
2(4)+6(1)=14
|
H H
| |
H --- C ---- C --- H
| |
H H
|
|
|
|
|
C2H4
see note
|
2(4)+4(1)=12
|
H H
\ /
C====C
/ \
H H
|
|
|
|
|
CH2O
|
|
|
|
|
|
Questions:
1. In the last 2 molecules you were asked to examine, there were multiple bonds present. How does the presence of a multiple bond (double or triple bond) affect your determination of the geometry of the molecule? Use C2H4 or CH2O to illustrate your discussion.
2. Although models cannot show this simply, for molecules like NF3 (whose structure you drew earlier), the actual bond angles may not exactly equal the angles predicted by VSEPR theory. For example, the F-N-F bond angles in NF3 are less than the 109.5° tetrahedral angle. Why do you think this m