Determining Polarity in a Substance
Polarity has to do with the spread of electronegative charge on an atom. In order to understand this, you have to understand Lewis structures and bonding.
Lewis Dot Structures
To determine polarity it is very helpful to draw out the Lewis dot structure first. In order to do these, there are a few rules that need to be observed first.
Rule #1: The Octet Rule
The octet rule simply means that an atom in the structure can only hold eight valence electrons total. The only exception to this rule is hydrogen, which only needs two electrons to become stable. It is important to note that not all structures will fully fill each atom.
How to Draw Lewis Structures
Rule #2: The Middle Atom
The atom that will be in the center of the figure will will the one that is the least electronegative. This is because it can effectively "pull in" the other atoms. The chart for electronegativities is below, while it is not usually required to memorize these, it is helpful to recognize the trend. Electronegativity generally increases going up and to the right of the periodic table.
Once the Lewis structure is done, it is now a matter of symmetry.
Let's look at the Lewis structure of CO2.
It is visually obvious that this structure is symmetrical. There are double bonds on each side of carbon and two lone pairs on each side as well. Since the polar bonds between carbon and oxygen cancel out, it makes the entire structure non polar.
A molecule will be non polar if it has non polar bonds, such as O2.
A polar bond is formed between atoms of different electronegativity. Polar bonds of equal polarity cancel each other out. To determine the difference between ionic, polar, and non polar bonds, calculate the difference between electronegativities and then use this scale.
Ionic: greater than 1.70
Polar: 0.40 to 1.70
Non Polar: 0.0 to 0.40
Polar and Non Polar bonds are covalent.
When there are lone pairs in a structure, things change. If the pairs are symmetrical as in CO2, they do not affect polarity. If they are not symmetrical, it can cause the substance to be polar. The best example of this is water.
H2O is polar because the lone pairs, which are each across from the bonds, do not cancel out the polar bonds. This causes the structure to be polar. If there were no lone pairs, water would be nonpolar. This proves the affect of lone pairs on polarity.
If you know anything about structural geometry, then you can recognize certain shapes of structures. Water is a bent structure (two bonds and two lone pairs), and bent structure will always be polar. Trigonal Pyramidal structures are always polar as well.