Re: how is the polarity of a molecule predic

Hi Caroline!

The polarity of a molecule can be predicted by knowing a couple of things:
*what is the molecule's shape?
*do the atoms in the molecule tend to pull electrons towards their nuclei, 
share them, or let them be pulled?

So, let's start with the first point: how do you know a molecule's shape?  
"VSEPR," or valence-shell electron-pair repulsion theory, allows you to 
predict molecular shapes based on the number and geometry of electrons in an 
atom's outermost valence shell. For example, carbon has 4 unpaired electrons 
in its outer shell, which arrange themselves in a stable tetrahedral 
structure.  Nitrogen has one electron pair + three unpaired electrons in its 
outer shell; these form a stable molecular structure called a trigonal 
pyramid (in other words, a triangle with a pyramidal base). There are some 
articles on VSEPR in MadSci Network archives, so I won't press 
this point too much further -- check out the archives if you need a VSEPR 
refresher.

Given a molecule's shape, then, how do you know its polarity?  Let's 
consider two carbon-based molecules, for simplicity's sake:

	methane (CH4)			chloroform (CHCl3)
           H                         H
           |                         |
       H - C - H                Cl - C - Cl
           |                         |
           H                         Cl

We know that the central carbon atom has its outer electrons arranged in a 
tetrahedron.  Now we need to think about the atoms bonded to the central 
carbon.  In methane, 4 bonds are formed with hydrogen atoms, in a 
symmetrical structure.  The molecule has no net polarity.  Chloroform has 3 
bonds with chlorine (two of them symmetrical; see the picture), and one with 
hydrogen.  Chlorine, like many of its neighbors on the right side of the 
periodic table, is very electronegative: it pulls electrons towards its 
nucleus, like so:

                     C .. Cl
                      ---->
           partial +       partial -
  ('loses' electron)      ('gains' electron)

Thus, it is a polar bond.  The two symmetrical chlorine-carbon bonds 
"balance out" this pulling, so that no net pull occurs.  The remaining 
chlorine-carbon bond is not "balanced out" by the less-polar hydrogen-carbon 
bond, resulting in a net polarity, with the "-" end towards the chlorine 
atom.

If you know which atoms tend to be electronegative (F, Cl, O, N, S), you 
will be able to predict which bonds are polar.  When two polar bonds are 
arranged equal and opposite (like the symmetrical Cl-C bonds in chloroform), 
no net polarity occurs. But, when polar bonds DON'T cancel, or if a polar 
bond is across from a nonpolar bond, a net molecular polarity ensues.  Try 
sketching out these molecules/bonds and convincing yourself of their 
polarity:

		POLAR					NOT POLAR
	water (H2O)				ethane (C2H6)				 
    ammonia (NH3)				oxygen (O2)
	ethanol (CH3CH2OH)			carbon tetrafluoride (CF4)

(for molecules with more than one central atom, like ethane or ethanol, it 
REALLY helps to draw the shape and consider the polarity and geometry of 
each bond).

Good luck,
Amanda
amandak@phy.ucsf.edu

Resources: http://
www.ultranet.com/~jkimball/BiologyPages/ (information about 
electronegativity is shown under "E", including a good graphic)
MadSci archives for "polarity, chemistry" and "VSEPR" has more in-depth 
answers that relate to this topic.