From: Marc Afifi (mafifi@redshift.com)
Date: Thu Nov 18 1999 - 10:49:18 PST
Message-ID: <38344A25.6BEF@redshift.com> Date: Thu, 18 Nov 1999 18:49:18 +0000 From: Marc Afifi <mafifi@redshift.com> Subject: Re: polar vs nonpolar molecules
Okay Geoff,
Here's my take on the nonpolar molecule question:
Since all dipoles in the molecule cancel out, there is no net attraction
nor repulsion. The reason for this can be understood by the same logic
that I used to explain the attraction that water molecules experience.
If you choose carbon tetrachloride to illustrate, then the polar C-Cl
bond has an electronegativity difference of only 0.4 (data from Brady
and Holum Chemistry, 2nd Edition, p. 283). Carbon's EN is 2.5 and
chlorine's EN is 2.9. It is the tetrahedral symmetry of the molecule
which causes the the dipole moments to cancel.
Now, let's analyze the situation where one chlorine atom is facing the
charged plate. In this case the plate would be positive to attract the
chlorine atom. But the plate would also be repelling the central carbon
atom since it too is partially positive. The repulsive force on the
carbon atom would be greater than the attractive force due to the
nearest chlorine atom because there is also an area of positivity on the
other three sides of the carbon atom at roughly the same distance from
the positive plate. On the other hand, there is a weak attraction due to
the other three chlorine atoms but they are at greater distances from
the plate. The net result is that these forces cancel out and there is
no net force of attraction or repulsion on the molecule which is why the
experiment doesn't work with nonpolar molecules.
What do you think?
-Marc
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