Molecular Shape and Polarity

[ERIKTORRESDisc3C]

How can you determine polarity based off of the shape of a compound? I could not understand exactly why Phosphorus trichloride was polar and not nonpolar when I encountered it on one of the practice midterms.

[Shirley_Zhang 3O]

One general rule to determine the polarity of a molecule is to see whether the bonding electron pairs are scattered evenly around the central atom. If they are evenly scattered, then the molecule is nonpolar. (The idea behind this guideline is that the difference between partial charges of surrounding atoms and central atom determines polarity.) Different elements have different electronegativity, thus, resulting in different strength of "pulling force" on bonding electrons from the central atom. In the simplest case when all the surrounding atoms are the same (meaning that they all have the same electronegativity), if there is a bonding pair in one direction and another bonding pair in a completely opposite direction, then this particular molecule is NONPOLAR.

Let's take a look at phosphorus trichloride (PCl₃):
It has tetrahedral arrangement except one bonding pair is replaced with a lone pair, so PCl₃ is a trigonal planar.
REMEMBER, polarity is determined by BONDING PAIRS. In this case, three bonding pairs are concentrated on one side, and there is not other bonding pairs to balance out on the other side. As a result, PCL₃ is POLAR.

Hope this helps :)

[Wang_Pong_Chan_1L]

When you are analyzing the molecular shape of a compound, if an atom does not have another atom countering its attraction, it is polar.