How To Tell if a Molecule Is Polar or Non-Polar?

These are problems using 3D molecules run in the application Jmol to help you visualize the molecule to determine if it 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).

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). If you click on the example molecules (where it says 3D view) below you'll get a better understanding of why some molecules are polar and some not.

Note: molecules with two atoms are not shown in these examples; they are always linear with sp hybridization. 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.

VSEPR Rules: Table of Molecular Geometry, Molecular Polarity Problems (with 3D solutions!).

VSEPR Rules:

Electron and Molecular Geometry On Central Atom. Click for Print View.

Electron Regions, shape, & hybridization

Bonding Regions

Lone Pairs

Electron Region Geometry

Molecular Geometry

Examples