A polar compound is a substance whose molecules have relatively large dipole moments due to non-uniform charge distributions. Typically, polar compounds contain one or more of the highly electronegative elements nitrogen, oxygen, or fluorine. Because of their large dipole moments, polar compounds generally have high boiling points. The most polar organic compounds are capable of forming hydrogen bonds, either between molecules or intramolecularly. Additionally, highly polar molecules will be more strongly retained on stationary phases such as silica gel during thin layer chromatography or column chromatography. An approximate ranking of organic functional groups from most polar to least polar is: sulfonic acids, carboxylic acids, amides, phenols, alcohols, amines, aldehydes, ketones, esters, ethers, aromatic halides, aromatic hydrocarbons, alkynes, alkenes, alkyl halides, and alkanes. As a general rule, any compound that has a larger dipole moment than that of a carbonyl group is termed polar whereas those with lesser dipole moments are considered nonpolar.