7 Differences between compounds - size of molecules and polarity

There are various factors which affect how volatile a compound is, that is, how easily it converts to a gas. One factor is the size of the molecule. When comparing molecules of similar composition smaller molecules tend to be more volatile than larger molecules. The alkane series demonstrates this - methane, ethane, propane, butane, pentane, and so on all have increasing boiling points.

A second factor is intermolecular forces – forces between molecules. The greater these are the less volatile a compound is (the higher the boiling point). This depends on the polarity of the molecules which is due the way electrons are distributed over the molecule - the extent to which the charge is uneven or separated determines the polarity of the molecule.

Most compounds containing only carbon and hydrogen are non-polar; however, those organic molecules also containing electronegative atoms such as oxygen and nitrogen tend to be polar. The organochlorine pesticides in this investigation contain chlorine atoms which are electronegative, but, in general, these molecules are relatively non-polar as there is little charge separation.

Another consequence of polarity is that if the polarity of the stationary phase and compound are similar then the compound will interact more strongly with the stationary phase so slowing the movement of the compound through the column.