How can you tell if a molecule will be ionic, polar, or nonpolar? For instance, oil is hydrophobic since the carbon and hydrogen bonds are nonpolar, and they don’t attract the polar qualities of H20. But why are some molecules ionic, some polar, and some nonpolar? Is there just a chart showing you what happens when the atoms come together…?
thanks for any help
It's usually dependent on a difference in electronegativity. The greater the difference, the more likely to be polar or ionic. Ionics are usually easy to spot as they have the greatest difference. Most non polars are CH bonds or something similar. Polars are covalent bond with a significant difference in electronegativity, though not as much as an ionic.
It has to do with the electronegativites of the atoms in the molecules. Electronegativity is an atom's tendancy to attract electrons toward itself. H20, for example, has oxygen with an electronegativity of 3.5 whereas hydrogen only has an electronegativity of 2.1, so the electrons within the bond will gravitate more towards oxygen. This gives oxygen a partial negative charge and hydrogen a partial positive charge. The two opposite charges within the molecule are known as poles, hence polar.
Oil as you said is a nonpolar molecule. It is made up of all hydrogens and carbons. The electronegativities being 2.1 and 2.5 respectively. The difference is not enough to create poles within the molecule so it is nonpolar.
Here is a chart of electronegativities:
http://www.standnes.no/chemix/periodictable/electronegativity-chart.htm
Generalty if the difference is greater than about 1 then the molecule should be polar. Hope that helps.
Yeah you'll save yourself a lot of trouble down the road if you start memorizing which elements have high/low electronegativity, at least for the important ones like C, O, N, S, and the halogens.
bump
Electronegativity, and shape. A linear molecule with equal dipoles in opposite directions isn't polar.
Just ask it!
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You have to sit down with the molecule and TALK to it. Maybe it will tell you, maybe not. If it's a Free Radical, you might not get any information at all, and it will just kick you in the nuts.
Just remember like dissolves like:
Polar solute molecules (i.e., ions or polar molecules) dissolve in polar solvents like water.
Non-polar solute molecules (i.e., molecules which contain preponderance of non-polar bonds) dissolve in non-polar solvents. These molecules are usually insoluble in water. However, some polar, small organic acids (i.e., house vinegar) will dissolve in water.
Ionic compounds are usually solids at room temperature and have high melting points. They dissolve only in very polar liquids like water.
If the molecule is very happy one moment and very sad that next moment it might be bi-polar.
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