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Re: [Phys-l] internal/external conservative/nonconservative forces!?!?



If I might add another minor footnote. A number of chemists do not consider ionic bonds to be chemical bonds. I know, it didn't make sense to me either, but I was informed of this in a review of my material by chemists.

Bill



On Dec 15, 2010, at 6:58 PM, John Clement wrote:

This is a description of covalent bonds, but not of ionic bonds. And a bond
is as others have questioned a construct of our model of how atoms stick
together. But the model even at the simplest level can make some
predictions such as the shape of a molecule by looking at orbitals. But
what we make in science are models and sometimes it is difficult to say if
something is a model or "reality". In the end reality is a judgment, and
not science. So is energy real? Or are forces real? Are fields real or
just mathematical abstractions? I don't think these questions have a unique
answer. You decide for yourself, but when presenting them to students there
may be a best practices answer.

But notice at the submicroscopic level "things" are not the same as at the
macroscopic level. So an atom is an abstraction because it has constituents
which are "fuzzy" and its size is a judgment derived from such things as how
close they approach. But it in no way is like a marble which we see a
discrete object. A bond would be "seen" as a pattern just as a orbital can
be seen as a pattern. A bond is just a real or unreal as an orbital. It is
a state of the system defined by the constituent atoms.

Chemists tend to think of bonds as being real because they deal with them
all the time. Just as physicists can visualize field lines and they seem to
be real, chemists visualize bonds. From the physics point of view a bond
would be just another state of the electron orbitals for a multi nucleus
system. The gravitational field pulls the object down towards the Earth,
and the bonds (fields?) pull the atoms together.

So a bond is like an unstretched rubber band, which has minimal energy. But
when you pull the atoms apart the bond stretches and energy is going into
the system. I suppose you could say that is the point where the bond has
energy in it, but the maximum energy stored is when the bond is no longer
apparent, or like a string of taffy has stretched so small you can't see it.
The term breaking bonds is most unfortunate because it conjures up the image
of a rubber band breaking and releasing energy. But bonds never break they
just stretch and weaken until they are no longer perceptible. Language is
important and the traditional language is often a barrier to understanding.

So bonds are models, but very potent models. They give the chemists and
students a way to visualize how atoms bond, and give at least qualitative
pictures of how this works. Students go into chemistry with very hazy ideas
about potential and kinetic energy, so bonds are a concrete visualization
that can be helpful. Working from QM descriptions of orbitals to explain
many chemistry effects is like doing kinematics of race cars using the
relativistic equations. It is possible, but not what you want to do.

John M. Clement
Houston, TX

Good question. I'll take a stab at it. Bonds are a configuration in
which separate orbitals (mathematical constructs) form a new hybrid
orbital, one that is energetically favorable. Bonding orbitals are a
constructive addition of separate wave functions and anti-bonding
orbitals are a destructive addition of separate wave functions. Not
sure if that helps matters any.



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