How can you tell the polarity of Covalent molecules using their shapes and individual bond polarity? Please explain thoroughly.
I understand why some smaller covalent molecules are nonpolar or polar based on the different molecular shapes (Ab, Ab2E2, Ab3, etc) but I don't know how to tell the molecular polarity of larger molecules like Ethyl alcohol or Glycine.
Covalent bonds are directional in nature. The individual bonds may be polar, or non-polar. Polarity of covalent molecules can be judged by inspecting these two aspects i.e. individual bond polarities and disposition (in space) of all the polar bonds and lone pair of electrons (if any). A vector summation (resultant) of all these moments (bond moment as well as lone pair moment), gives an idea about the overall polarity (or otherwise) of a covalent molecule. Finding out the resultant for simple molecules through a mathematical operation is a trivial job.
Theoretically the same rules can be extended to find out polarity of relatively larger molecules like Glycine (NH2CH2CO2H) or Ethyl alcohol (C2H5OH). Both of these molecules are polar. Glycine being a special case where clear positive and clear negative pole exists in two part of the same molecule, through internal proton transfer. This is called a zwitterionic form (NH3^+CH2CO2^-). A popular shortcut to cumbersome mathematical calculations is provided by the concept of functional groups in organic molecules. For functionalized molecules like in the examples given above, the functional group is responsible for most of the polarity, and just by looking at the polarity and disposition of the functional group(s), the overall molecular polarity can be predicted with a fair degree of confidence. For still larger molecules one has to consider the relative orientation of all the functional groups, as well as other structural fragments of notable polarity. A vector resultant of all these fragments gives overall polarity.