Lewis Structures and Molecular Geometry

Lewis Structures and Molecular Geometry

Lab Activity

Procedure:

Part A:

1.  Write the formula for each molecule or polyatomic ion listed in Data Table A. Some of the formulas have been filled in for you.

2.  Count the number of valence electrons supplied by each atom in the formula. Determine the total number of valence electrons and record this number in Data Table A. In the case of polyatomic ions, add on electron for each negative charge or subtract on electron for each positive charge.

3.  Draw Lewis structures for each molecule in part A

Part B:

4.  Examine the molecular models A – K. For each model, identify the number of bonding pairs, the number of unshared pairs and the total number of electron pairs around the central atom. Record this information in Data Table B

a.  NOTE: In the case of double or triple bonds, count all of the electrons involved in the bond as ONE PAIR

5.  Sketch the three-dimensional arrangement of valence electron pairs around the central atom in Data table B

6.  Use the following terms to describe the molecular geometry for each model:

a.  Linear, Bent, Trigonal pyramidal, tetrahedral, trigonal bipyramidal, octahedral

b.  Record the molecular geometries for each molecule in the data table

Part C:

7.  Compare the lewis structures in Data Table A to the molecular Geometry sketches in Data Table B. Count the number of valence electron pairs (see step 4) around the central atom and predict the molecular geometry for each molecule or ion in Data Table A.

8.  In Data Table A, Draw a 3-D sketch of the molecule or ion in the space below its name and identify its molecular geometry.

Sample 3-D drawings

H2O could be shown as this while H3O+ could be shown as this

Data Table A

Name / Molecular
Formula / Valence Electrons / Lewis structure
Methane / CH4
Chlorine (diatomic!)
Sulfur Hexafluoride
Ammonia
Ammonium ION
Hydrogen Sulfide / H2S

Data Table A (con’t)

Name / Molecular
Formula / Valence Electrons / Lewis structure
Sulfite ION
Arsenic Pentafluoride
Phosphorus Trichloride
Carbon Tetrachloride
Thiocyanate ION / SCN-1
Sulfate ION

Data Table B:

Model / Bonding Pairs* # / Unshared Pairs* / Total Pairs / Sketch and Molecular Geometry
A
B
C
D
E
F
G

*Count the bonding and nonbonding PAIRS of electrons around the central atom ONLY

# In the case of double or triple bonds, count them as ONE PAIR

TEACHERS NOTES – Model Kits

Model / Bonding Pairs* # / Unshared Pairs* / Total Pairs / Sketch and Molecular Geometry
A / 2 / 0 / 2 / CO2 (double bonds) Linear
B / 4 / 0 / 4 / Tetrahedral
C / 3 / 1 / 4 / Trigonal pyramidal
D / 2 / 2 / 4 / Bent
E / 1 / 3 / 4 / Linear (terminal)
F / 5 / 0 / 5 / Trigonal Bipyramidal
G / 6 / 0 / 6 / Octahedral

*Count the bonding and nonbonding PAIRS of electrons around the central atom ONLY

# In the case of double or triple bonds, count them as ONE PAIR