Polar Molelcules vs. Non-Polar Molecules?

Question

Ok. I'm a bio. major who has had more chemistry classes than I care to count.
I know it seems like this is an elementary chemistry question that I should have figured out a very long time ago, but for whatever reason, understanding this concept is beyond me.

Could someone please explain to me, in laymans terms, how to look at a molecule, or a chemical formula, and tell if the molecule is polar or non-polar.

Thanks in advance...

Answer 1

There are different grades of polarity. Polarity is hard to tell by simply looking at a molecule, since there is a lot of spooky stuff happening at the quantum level. Some polar molecules are more polar than others. If you have an -OH group in an organic molecule (polar) then you have a somewhat polar molecule. Not as polar as say, water. Water is extremely polar. Polar molecules include

NaCl (salt), H2O (water)

Non-polar molecules include

Cl2 (chlorine gas), H2 (natural hydrogen), O2 (atmospheric oxygen)

However some molecules you would expect to be extremely polar based on oxidation states are NOT, like HCl (hydrochloric acid) where the Cl shares electrons with its hydrogen neighbor. So generally by looking at a molecule you can't really tell. But there are some guidelines.

1. Diatomic bonds (as found in naturally occuring oxygen, nitrogren, etc, i.e. O2, N2) are non-polar.
2. Organic molecules are generally non-polar (there are exceptions: some soaps, ketones, alcohols, organic salts)
3. Non-organic salts are polar
4. An alcohols polarity DECREASES when you add more chains. I.E. methyl alcohol is MORE polar than ethyl alcohol, which is more polar than isopropyl alcohol. Extremely long chain alcohols even with an -OH group are essentially non-polar.

Answer 2

Lets take water. H2O. If the atoms were in a straight line, the charges would be balanced with oxygen in the middle.
The atoms are not in a straight line, however. There is an angle of 104 degrees. This makes the water molecule look like a V.
Oxygen attracts electrons much more than hydrogen, so the oxygen atom ends up negatively charged. The hydrogens end up positively charged. The oxygen makes the point of the V negative, and the two hydrogens make it's arms positive.
The water molecule has a positive side and a negative side. This makes it a polar molecule.
Normal Hexane is a straight chain of carbon, each carbon carbon bond a single bond, and two hydrogen atoms on each of the inner carbons, and three hydrogens on each of the end carbons. The charges are evenly distributed. It is non-polar. there is no strong negative or positive area on this hexane molecule.
Soap works because it is part polar, and part non polar. The polar part attaches to water molecules, and the non-polar to the grease (oil) which is similar to the hexane we just discussed. It is a hybrid, and allows polar water to dissolve non- polar grease.
The rule in general is like dissolves like. Polar dissolves polar, and non-polar dissolves non- polar.
In more complicated molecules, look for areas where there is a lack of symmetry. Different atoms attract electrons differently, some more (electronegative)
and some less (electropositive). If there is a lack of symmetry, if one part doesn't mirror another, then there will be areas of different charge, and the molecule will be more or less polar.
It is best to see a three dimensional model of a molecule, to determine it's symmetry, or lack of symmetry.
Fluorine has the greatest attraction for electrons. Each atom has a certain rating on a scale of electronegativity-electropositivity.
Imbalance of this, over a molecule, will make it polar.
Even and symmetrical distribution of atoms over the molecule will make it non-polar.
I tried to simplify it. Please write and tell me if this helps.

Answer 3

There are a few simple things to look for. First, if the molecule is symmetric, then it will be nonpolar. If it is not symmetric, then you start looking at the composition, as well as the structure. You want to look for electron donors and electron acceptors (you will know which it is by the electronegativity). Polar moleculs tend to have oxygen that "pulls" electrons away from other elements. Water is the easiest example. Oxygen is much more electronegative than hydrogen, so it is going to pull the electrons away from the hydrogens. As a result, the oxygen end of water is slightly more negative than the hydrogen end, and thus you have a polar molecule. Alcohols work in a similar manner.

Answer 4

How good are you at determining molecular geometries? Once you know the molecular geometry, it is easy to determine if a molecule is polar or not. Molecules that have an odd number of lone pairs are polar. An example would be water. Silicon dioxide is also polar, because silicon has a lone pair, and only one of the oxygen's has a double bond. Molecules that are symmetrical are nonpolar. Methane would be an example. Once you learn the small molecules, predicting the polarity of large molecules will become fairly simple.

Answer 5

You need to think about relative charge distribution on the molecule; Water, for instance, is highly polar, because the oxygen atom is sort of "at one end". Organic chain molecules with a functional group at one end will be polar too, but the longer the chain, or the more evenly distributed the functional groups, the less polar they will be.

This link might help, good luck!

Answer 6

non polar molecules are symmetrical, with equal "pull" coming from all ends of the compound

for example CH(4) is non polar

polar molecules have one end pulling harder than others

for example water has extra charge on the oxygen and the hydrogens are not pulling as hard, so it is polar.