Terminologies

Dear all,

I understand that some of you may be confused about the terminologies polar/nonpolar/hydrophilic/hydrophobic. These terminologies are probably more Chemistry-based than Biology-based so I may not be an expert in this, but let me try my best to clarify these terms. I think you will learn more about these terms in Chemistry so maybe you will have a better idea then. The examples I have used below are from Wikipedia.

Polarity is dependent on (1) the difference in electronegativity between atoms in a compound and (2) the asymmetry of the compound's structure.

(1) The main reason why a molecule is POLAR is due to the presence of a large electronegativity difference between its atoms. This leads to unequal sharing of electrons in a covalent bond.

For example, water (H2O) is a polar molecule due to the unequal sharing of electrons between oxygen and hydrogen, as oxygen is more electronegative than hydrogen and will hence pull shared electrons towards itself more. Due to the polar nature of the water molecule itself, polar molecules are generally able to dissolve in water. Hence, polar and nonpolar molecules are also called hydrophilic and hydrophobic molecules, respectively.

On the contrary, oxygen (O2) is a nonpolar molecule as the oxygen atoms are of equal electronegativity. Methane (CH4) is also a nonpolar molecule because carbon shares the electrons with hydrogen almost equally.

(2) The shape (you can google "VSEPR" to see all the possible shapes of molecules) of the molecule also determines whether a molecule is polar or nonpolar. A molecule is POLAR if there is asymmetrical arrangement of polar covalent bonds.

A phosphate group (PO4) has a tetrahedral (asymmetrical) shape, so there is an imbalance between the 4 oxygens and creates and stronger pull to one side of the molecule and hence making it polar.

On the contrary, boron trifluoride molecule (BF3) has a trigonal planar (symmetrical) arrangement of three polar bonds at 120°. This results in a more equal distribution of electrons in the molecule so it is nonpolar.

Hope this helps.