You should already know that an ion is a charged atom or molecule. The charge is due to either an excess of electrons (which gives the particle a negative charge) or too few electrons (giving the particle a positive charge). Once you know that a given substance is in an ionic form, two important facts should be understood:
Ions are not the only hydrophilic molecules as many other substances are water soluble. This other group of hydrophilic molecules are polar molecules. This does not mean that they are cold substances but that the molecules have dipoles. A dipole is a partial electrical charge on a portion of a molecule that is due to the unequal sharing of electrons by the atoms of the molecule. Thus polar molecules have partial positive and partial negative charges. Because of the dipoles, when one mixes polar molecules together, the negative dipole of one molecule and the positive dipole of another will be weakly attracted to one another. This weak attraction is called a hydrogen bond and the formation of this bond allows for polar molecules to dissolve in one another. Since water is a polar molecule, other polar molecules will dissolve in water (thus polar molecules are usually hydrophilic and are lipophobic). Molecules that are not ions or are not polar are termed nonpolar. Nonpolar molecules do not dissolve in water as they cannot form hydrogen bonds (thus are hydrophobic) but do dissolve in lipids or fats (lipophilic).
As an example, imagine a molecule that contains two carbon atoms attached to one another by a covalent bond (a bond in which at least one pair of electrons is shared by both atoms). Both carbon atoms have an equal affinity for electrons so the electrons will be shared equally by both atoms. This would be a nonpolar molecule. Now suppose instead that we have a molecule that contains a carbon atom attached to an oxygen atom by a covalent bond. It turns out that oxygen has a greater affinity for electrons than the carbon atom. Therefore, the shared electrons tend to stay around the oxygen atom more than the carbon atom. Since electrons carry a negative charge, the oxygen atom would have a partial negative charge and, conversely, the carbon would have a partial positive charge. These partial charges are dipoles and they make the molecule polar.
You should remember that oxygen and nitrogen have relatively high affinities for electrons, and therefore oxygen-carbon and oxygen- hydrogen bonds as well as nitrogen-hydrogen bonds are all polar bonds (with the oxygen or nitrogen partially negative and the hydrogen or carbon partially positive). Carbon and hydrogen have similar electron affinities thus carbon-hydrogen bonds are nonpolar (as are carbon-carbon bonds). Thus to a first approximation, a biochemical molecule that contains large amounts of oxygen or nitrogen would most likely be polar (and thus hydrophilic) while one that contains primarily hydrogen would be nonpolar (and thus hydrophobic). It is also good to remember that carbon is the backbone of all biochemical molecules.
Try your hand at a brief quiz on polarity.
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