From: Steven Eiger (eiger@montana.edu)
Date: Fri Nov 19 1999 - 23:21:36 PST
Message-Id: <l03102802b45bfabe8445@[153.90.241.107]> Date: Sat, 20 Nov 1999 01:21:36 -0600 From: Steven Eiger <eiger@montana.edu> Subject: Re: pinhole Re: polar vs nonpolar molecules
Marc,
I think that much of what you say is roughly correct, but incorrect. In a
nonpolar molecule we can have true inductive forces, not ones created by
reorienting molecules out of a random arrangement. The electrons can be
pulled slightly by a static charge close by which would lead to an
inductive effect. This is close to the rationale for Van der Waals Forces,
or hydrophobic bonds, or London Dispersion Forces which create the tiny but
perceptible forces between strings of hydrocarbons; it is a function of the
orchestration of electron clouds, small but there. Thus comparing the
macroscopic diversion of water flows versus nonpolar flows one would
conclude that this phenomenon does not occur in nonpolar molecules, but I
bet it is there. Eiger
>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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Steven Eiger, Ph.D.
Departments of Biology and the WWAMI Medical Education Program
Montana State University - Bozeman
Bozeman, MT 59717-3460
Voice: (406) 994-5672
E-mail: eiger@montana.edu
FAX: (406) 994-3190
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