How To Tell if a Molecule Is Polar or Non-Polar?
Step 1: Draw the Lewis structure. Note the number of electron regions around the central atom,
and of these which are bonding or lone pairs (non-bonding pairs)
Step 2: Use this info to determine the 3D geometry of the molecule. You do this by
remembering "VSEPR". Valence Shell Electron Pair Repulsion theory; around the central atom
all regions of electrons repel each other to get as far away from each other as possible while
pivoting around the central atom. Note that double bonds and triple bonds count as a single
region of electrons.
Step 3: Determine if the molecular is polar or non-polar - a molecule is (i) non-polar if the
charge distribution is symmetric and (ii) polar if the charge distribution is asymmetric (not
symmetric). This is where we look at dipoles (polar vs. nonpolar) for individual pairs of bonding
electrons.
The Decision Process:
After you draw the molecule in 3D representation using VSEPR rules, if the molecule has
symmetry around the central atom, the bond dipole moments will "cancel out" (like pulling in
opposite directions) and the molecule will therefore be non-polar.
However, if the molecule is asymmetric, the bond dipole moments won't "cancel out" and the
molecule will have a net dipole moment and the molecule is therefore polar.
To really understand how to do this, the Lewis structure is only the first step. You need to
consider the molecule in 3D (three dimensions). You may make the following molecules using
your molecular modeling kits to accomplish this.
Note: molecules with two atoms are not shown in these examples; they are always linear. For
two atom molecules, if the atoms are the same, the molecule is non-polar molecule; if the atoms
are different, the molecule is polar.
What good is this?
The polarity of a molecule will tell you a lot about its solubility, boiling point, etc. when you
compare it to other similar molecules. Water, for example, is a very light molecule (lighter than
oxygen gas or nitrogen gas) and you might expect it would be a gas based on its molecular
weight, however the polarity of water makes the molecules "stick together" very well. And it's a
good thing, because if water was not so polar, we would certainly not be here.
For each of the following:
1.
2.
3.
4.
Draw the molecule in 3D and name the shape.
Calculate the ΔEN(s)
Determine if there are any dipoles present.
Determine (if there are dipoles) if the charge is evenly or unevenly distributed, and
therefore whether the molecule is overall nonpolar or polar.
Click on the molecule's name to see the answer, but first try to do it yourself!
(Note: Trigonal pyramidal is listed as tetrahedral. This is because it really is just a variation on
the tetrahedral shape, and so is sometimes referred to as such. I expect you to differential
between trigonal pyramidal and tetrahedral.)
1. AlCl3 - Aluminum
Trichloride
2. CS2 - Carbon Disulfide
3. BeI2 - Beryllium Diiodide
4. NOCl - Nitrosyl Chloride
5. PO(OH)3 - Phosphoric Acid
6. SO2Cl2 - Sulfuryl Chloride
7. NOCl - Nitrosyl Bromide
8. BCl3 - Boron Trichloride
9. SiH4 - Silicon Tetrahydride
10. BeBr2 - Beryllium
Dibromide
11. CH2O - Formaldehyde
12. NH2Cl - Chloramine
13. CH4 - Methane
14. SO2 - Sulfur Dioxide
15. AlF3 - Aluminum
Trifluoride
16. NH3 - Ammonia
17. SeH2 - Hydrogen Selenide
18. H2O - Water
19. CO2 - Carbon Dioxide
20. SCl2 - Sulfur Dichloride
21. NO2F - Nitryl Fluoride
22. CSe2 - Carbon Diselenide
23. CCl4 - Carbon Tetrachloride
24. AlBr3 - Aluminum
Tribromide
25. BeCl2 - Beryllium
Dichloride
26. SO3 - Sulfur Trioxide
27. PCl3 - Phosphorus
Trichloride
28. BeF2 - Beryllium Difluoride
29. BF3 - Boron Trifluoride
(Source: http://www.tutorhomework.com/Chemistry_Help/Molecular_Geometry/Polar_Or_Nonpolar.html#problems )