Question

I DO NOT have a specific question, my question here is GENERAL

HOW TO DETERMINE IF A COMPOUND IS POLAR OR NONPOLAR?

I know how to figure about the number of valence electrons and I KNOW how to draw lewis structure, but then I just mess every thing up. I want to know how to determine if it's polar or not.

for example: sicl2f2, co2, xef2, xeo4 ... I KNOW how to draw the lewis structure, but this I CANNOT decide whether they are polar or not.

so please if someone knows or can explain an easy way to figure it. the book keeps mentiong symmetric and asymmetric but it just did not help

Answer 1

If the molecule is arranged in such a way that the electrons are very evenly distributed, then the molecule will be non-polar. If you can draw a molecule's Lewis diagram, it's simple to figure out whether it is or isn't polar:

For molecules that have a single central atom (like H2O, CH4, NH3, etc).

If the central atom has one or more lone pairs of electrons, the molecule is most likely polar.

If the central atom has no lone pairs and all of the outer atoms are identical, the molecule is most likely non-polar.

If the central atom has no lone pairs and the outer atoms are NOT identical, the molecule is most likely polar.

For example, you can predict a water molecule's polarity because the central oxygen atom has two lone pairs of electrons on it. CH4 has no lone pairs on its central carbon atom, and all of its outer atoms are identical (H), so it's definitely non-polar; CH3Cl has no lone pairs on its central carbon atom, but one of the hydrogen atoms has been replaced by a chlorine atom. Since the chlorine atom is going to draw electron density toward its side of the molecule, CH3Cl is polar.