Molecular Polarity
How do we know if one molecule will be attracted to another molecule?
Why?
Did you ever wonder how molecules of a substance stay close together? Why does water form
droplets or support things on its surface? Electrons may be distributed evenly or unevenly throughout
a molecule, which creates partial charges at different parts of the molecule. These partial charges on
one molecule often interact with the partial charges of a neighboring molecule. In this activity you will
learn how to determine molecular polarity and placement of partial charges molecules.
Model 1 – The Molecules
POLAR MOLECULES:
Tetrahedral
(CH2Cl2)
Trigonal Pyramidal
Tetrahedral
Tetrahedral
(NH3)
(CH2Cl2)
Bent
(CH3Cl)
(H2O)
NON-POLAR MOLECULES:
Tetrahedral
Trigonal Planar
(CCl4)
Tetrahedral
(BCl 3)
(CH4)
Trigonal Planar
Linear
Bond Type Guide:
C – Cl = Polar Covalent
C – H = Polar Covalent
C = O = Polar Covalent
(CO2)
BH2At
N – H = Polar Covalent
O – H = Polar Covalent
B – Cl = Polar Covalent
B – H = Non-Polar Covalent
B – At = Non-Polar Covalent
1. a) List the chemical formula for all of the polar molecules in Model 1.
b) List the chemical formula for all of the non-polar molecules in Model 1.
2. Using the Bond Type Guide, which of Model 1’s examples contain polar covalent bonds?
3. Which examples contain non-polar covalent bonds?
4. Do all non-polar molecules contain only non-polar bonds?
5. Five of the molecules in Model 1 are tetrahedral. Are all of these molecules polar?
6. a) For the non-polar tetrahedral molecules, how many types of atoms are bonded to the central
atom?
b) For the polar tetrahedral molecules, how many types of atoms are bonded to the central atom?
c) What is the difference between the arrangement of atoms in the non-polar tetrahedral molecules
as compared to the polar tetrahedral molecules.
7. Insert the dipole arrows, difference in electronegativity, and partial charges for all polar covalent
bonds.
8. Of the examples, which have arrows of the same magnitude that point in exact opposite directions?
9. Spend some time building the examples of Model 1 with molecular modeling kits. Double check
your answer to question #8..
10. a) If arrows with the same magnitude point in exact opposite directions, what happens to the “pull”
on electrons?
b) How are the electrons distributed in such a molecule?
READ THIS!!
Many times there are scenarios where you have arrows of the same magnitude, but they don’t point in
opposite directions. This can be tricky. A molecule can still be non-polar if the overall “pull” on
electrons cancels out because the bonds (and therefore arrows) are evenly spaced in terms of 3dimensional geometry.
11. Write the chemical formula for the molecule(s) that show the concept mentioned in the Read This.
12. Describe the characteristics of a molecule that make it non-polar.
13. Which examples in Model 1 have bonds evenly spaced in terms of 3-D geometry, but have
different magnitudes on the arrows?
14. What would happen to the electron distribution in these molecules?
15. a) How many examples have polar covalent bonds that are not evenly spaced in terms of
geometry?
b) Write the formula for each one.
16. Will the “pull” on electrons be even in these molecules? Why or why not?
17. Describe the characteristics of a molecule that make it polar.
READ THIS!!
Bonds aren’t the only thing that can have a partial charge. Entire molecules can have a partial charge
too. When a particular bond is polar covalent, the electrons are unevenly distributed towards one side
of the bond compared to the other. Therefore, there is a partial negative and a partial positive end to
the bond. A molecule works the same way. The side of the molecule that has more “pull” on the
electrons becomes partially negative, and the other side become partially positive.
18. If a molecule is polar, do all parts of the molecule have the same partial charge?
19. Insert partial charges on the following molecules (that are polar), not the specific bonds.
Extension questions
1. Examine the following molecules. Draw the structure in 3-dimensions, identify each bond type,
explain the electron distribution, and determine if the molecule is polar or non-polar. Be very
thorough and show all work.
CHF3 , SiO2 , NCl3 , H2S
2. Clearly show whether PCl5 is polar or non-polar.
3. On a test, a student claimed that PH2Cl3 could be either polar or non-polar. The teacher gave the
student full credit. Explain why, making sure to draw and reference a molecular drawing in your
explanation.