Download Activity: Molecular Geometry

Activity: Molecular Geometry
Name _____________________________________ Period ____ Group_____
Go to this website: http://phet.colorado.edu/en/simulation/molecule-shapes-basics
Click on the green "Run Now" button.
1. Start with a central atom (purple), with a single grey atom attached. If your model has
more than one atom attached, remove the others by clicking on the red X in the upper
right hand corner.
2. When you add a second atom, by clicking on the single bonded atom in the upper
right hand corner, what angle do the two grey atoms make?
3. Add a third bond; what is the angle now?
Is this molecule flat or three dimensional?
4. Add one more atom; how does this change the molecule? (Two answers!)
5. Check the "Options" box to show bond angles. Are all bond angles the same, or are
they different? Click on the "name" box in the lower left to see the name of this shape:
6. Remove one of the bonds, and determine the shape of the molecule with three bonded
atoms:
7. Choose the "Real Molecules" tab, and predict the shape and bond angles of:
BeCl2:
BF3:
CH4:
Check the "Name" and "Bond Angle" boxes to check your answers.
8. Go back to "Model" and "Remove All" bonded atoms. Then add two double-bonded
atoms. How is this shape different from your molecule in #1? (HINT: check the bond
angles and molecular geometry).
Now, go to this website: http://phet.colorado.edu/en/simulation/molecule-shapes
9. Select the "Real Molecules" tab; uncheck "show lone pairs."
10. Predict the shape and bond angle in H2O:
Check "show bond angles," and check "name molecular geometry."
11. Now, report its actual shape and bond angle:
12. Why are they (possibly) different than your predictions? Check the "show lone pairs"
box, and describe what you see:
13. Rotate the molecule by holding down the mouse and swiveling it. Re-examine the
CH4 molecule (pull-down menu). How do bond angles in H2O differ from those in CH4?
14. How many electron pairs are there around the C atom in CH4? (Remember that each
bond is a pair.) How many around the O in H2O?
Realize, then, that the 4 electron pairs around the central atom in H2O and in CH4 are
not much different, except that some are unbonded (lone) in H2O. This accounts for the
differences in shape and bond angle, since LONE PAIRS REPEL just like bonding pairs;
in fact, they repel even more than bonding pairs!
15. Look at the NH3 molecule; check and uncheck the "lone pair" box as necessary.
What similarities do you see to H2O and/or CH4? What differences?
16. How does the bond angle in NH3 compare with that of H2O & CH4? Is this molecule
2- or 3-dimensional?
17. Draw the Lewis Structure for each of CH4, NH3, and H2O. Be sure to include the lone
pairs!
18. How many electron pairs around each central atom in CH4/NH3/H2O? How many of
these pairs are used in bonding? Refer to table 12.4 (p. 389). How do the lone
pairs/bonding pairs affect the shapes?
GO TO: http://phet.colorado.edu/en/simulation/molecule-polarity
Select "Two Atoms," view "Bond dipole," and "Partial charges." Select either "Electrostatic
Potential" OR "Electron Density." Electric Field off. Set both atoms to medium
electronegativity values.
19. Decrease the electronegativity value of ONE atom, and record what happens to the
electrostatic potential/electron density (HINT: arrow & partial charges)
REALIZE THAT THESE STEPS ARE NOT POSSIBLE IN REAL LIFE—WE CANNOT
CHANGE THE ELECTRONEGATIVITY OF AN ELEMENT!
20. Increase the other atom's electronegativity, and record your results.
21. Turn on the electric field and describe how the molecule orients itself paying
attention to the partial charges.
22. Reverse the electronegativity values of the atoms and record your results.
Select "Three Atoms," and check "Bond Dipoles" and "Molecular Dipoles." Set all atoms
to medium electronegativities.
23. Decrease the electronegativity of C to the minimum value and record your
observations of both bond and molecular dipoles. Remember that the head of the arrow
points in the negative direction.
24. PREDICT what would happen if you were to minimize the electronegativity of atom A
and C at the same time, leaving B alone.
25. Try minimizing A and C while leaving B alone, and record your results.
26. Now turn on the Electric Field, and record your observations in detail: how does the
molecule respond to the electric field specifically?
Finally, select "Real Molecules," and check both (bond and molecule) dipoles, "atom
electronegativites," and "Atom Labels."
27. Describe the bond polarities and molecular polarities of the following molecules:
MOLECULE
H2
O2
H2O
HF
BF3
CO2
NH3
CH4
CH3F
BOND POLARITY (grey arrows)
MOLECULAR POLARITY (yellow arrows)
28. What can you conclude about molecular polarity from your analysis of the molecules
in #27? (HINT: compare the dipoles of CH4 and CH3F.)