Re: [Phys-L] Energy & Bonds

[John Denker <jsd@av8n.com>]

In the context of
  http://www.av8n.com/physics/img48/ion-energy.png
  http://www.av8n.com/physics/img48/ion-force.png

On 11/15/2013 10:52 PM, Bruce Sherwood wrote:
> Lest there be any misunderstanding, note that JD is plotting potential
> energy for (charged) ions bound to each other, not neutral atoms bound to
> each other. For the latter case, a well-known approximation is the Morse
> potential energy (http://en.wikipedia.org/wiki/Morse_potential) for which a
> harmonic oscillator well fits over a much wider range of distances than it
> does for the two-ion situation.

1) This thread has been primarily about
  a) The making and breaking of chemical bonds, and
  b) Modeling bonds by springs.

2) The fact is, (a) and (b) are inconsistent.  A Hooke's 
 law spring cannot begin to explain bond-making and 
 bond-breaking.

3) Nothing anybody says about the Morse potential -- or
 any other potential -- is going to change fact (2).

 Whatever part of the potential agrees with Hooke's law 
 is not consistent with bond-making and bond-breaking.

 Conversely, whatever part of the potential explains 
 bond-making and bond-breaking is not consistent with
 Hooke's law.

4) Nothing anybody says about ions or non-ions is going
 to change fact (2) or fact (3).

 The harmonic oscillator is pretty much harmonic, whereas
 bond-making and bond-breaking is wildly anharmonic, no
 matter how much (or how little) ionic character the bond
 has.

5) Ions exist in the real world.  Some chemical reactions
 involve ions.

 Even if we start with a molecular bond with little or
 no ionic character, if the bond is broken so as to
 produce ions, /then/ the Coulomb interaction survives
 long after the covalent bond dance has become negligible.
   At even-larger distances the Coulomb interaction will
   presumably be screened, but even then, you can't begin
   to understand screening unless you start with the
   Coulomb interaction.

 People should be allowed to ask about Coulomb's law if
 they want. 

=============

More generally, we should choose our models to fit the
facts, not the other way around.  We should not become
so enamored with a particular model that it prevents us
from understanding stuff we want to understand.

The spring model is just lovely for describing part of
the elephant ... but not the whole elephant.

  http://www.av8n.com/physics/img48/ion-energy.png
  http://www.av8n.com/physics/img48/ion-force.png