Ask A Scientist© Chemistry Archive

name        Simon
age         14

Question -  How does VSEPRT (Valence shared electron pair repulsion
theory)affect the structure of molecules?
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Dear Simon,
VSEPR theory does not affect the structure of molecules;
rather, it is used to PREDICT the structures of molecules.
Basically one figures out the Lewis Dot structure of a molecule
and counts the total number of structural electron pairs attached
to any one atom (either as lone pairs or as bond pairs).
The number of structural pairs, as well as how many of
each kind there are, can be used to predict the arrangement
of structural pairs around the atom.

You can read about VSEPR theory in any college-level
freshman chemistry text and many high school chemistry
texts. If you have any specific questions after doing
some reading, I'd be glad to help.


best regards,
prof. topper
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The flip but quite correct answer is that the theory does not make one bit
of difference to the structure of the molecules.  The molecules are what
they are, and the theory is what we think they are.  No matter what we think
they are, they will keep on being exactly what they are.

As for what VSEPR theory says about the structure of molecules, it's quite
simple.  It says that valence electron pairs will repel each other, and will
orient themselves around the central atom in such a way as to keep the
greatest distance from each other.  Furthermore, it states that 'lone pairs'
have more effective charge than bonding pairs (because the electrons are
closer to the central atom), so the greatest repulsion will be between two
lone pairs, and the least between two bonding pairs.

This theory works pretty well for many compounds.  It is, however, not
perfect.  For instance, it does a good job rationalizing the
near-tetrahedral H-O-H angle in water, but it doesn't explain why the H-S-H
angle in hydrogen sulfide is much closer to 90 degrees.  As I said earlier,
the molecules don't care what the theory predicts.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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Rather, VSEPR is a set of empirical rules that assigns a certain angular
volume to  bonds of molecules that possess a central atom of symmetry. This
volume depends upon the atoms involved, the valence shell(s) involved, the
relative electronegativity of the atoms, and the number of non-bonding lone
pairs of electrons. The rules provide reasonably accurate estimates of the
structure of such centro-symmetric molecules.

The method is described clearly, and in detail, by its inventor, R.J.
Gillespie in the Journal of Chemical Education, vol. 40 p.295(1963);
ibid.,vol. 47, p.18(1970); and
vol. 51, p. 376(1974) as well as a text MOLECULAR GEOMETRY by the same
author.
Vince Calder
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