Why are the molecules of hydrocarbons nonpolar?

This is not the case in the hydrocarbon tail. The electronegativity of hydrogen and carbon are very similar, so the electron cloud is distributed evenly over the two atoms. Carbon-hydrogen bonds are said to be non-polar because they do not have positive and negative poles within themselves. Hope this helps. 

Because hydrocarbons consist only C-H bonds, which are non-polar. When you look at the table of electronegativity (Pauling scale), you see, that the difference between the electronegativities of carbon and hydrogen is much smaller than that of nitrogen and hydrogen.

Hope the image of the frequency formula