Re: Bohr Theory

[Jim Diamond <jimd@linfield.edu>]

On Fri, 14 Nov 1997, Dr. William Newbolt wrote:
> 	I also wanted to say something about the post about why 
> Bohr Theory works.  It seems to me that it works as well as 
> it does because it is basically a semiclassical treatment 
> of an electrostatic attraction that creates a set of bound 
> states, which is basically correct.  The only thing wrong 
> with it is that the connection between these states and the 
> angular momentum of the system is too simple.  Not too 
> unexpected in view of the complexity of the view of the 
> role of angular moment in the theory of hydrogen.    

I enjoy teaching the Bohr model in my physical chemistry class. It is
remarkable that such a simple model can accurately produce the Rydberg
constant. 

I do not agree that it is successful in any other measure. The intensities
of transitions within the hydrogen atom are not correctly predicted (it is
not even clear how this would have been handled within the model, since
Maxwell's equations had to be tossed out de facto). There is no hint of
anything like selection rules. Nor are expectation values of properties
such as <r>, <r^2>, etc., correct. In addition, the position and momentum
probability distributions differ considerably from the quantum mechanical
distributions, so that there is at least a basis for arguing that the Bohr
model disagrees substantially with a far more successful theory (quantum
mechanics). 

The angular momentum quantization condition
	Integral (over closed path) p dq = n * constant
was in existence prior to the Bohr model, so it was not that for which
Bohr deserves acknowledgement.

In my opinion, the reason we should teach the Bohr model of the atom, and
the reason why Bohr deserves such acclaim for his model, is the
extraordinary effort which Bohr made to create a model whose very
assumptions went deeply against his fundamental understanding of physics.
He very clearly was concerned with creating a model which forced the
"laws" of physics to agree with nature, even at the expense of sacrificing
large parts of the existing physical picture of the universe. His model
was the culminating synthesis of many ideas now firmly rooted in quantum
mechanics, but which must have been almost completely mysterious a century
ago.

It is definitely worth reading Bohr's words on the subject. I think it is
possible to read in his words some of the anguish that must have
accompanied the many years of work on the hydrogen atom problem. It is
also worth pointing out that despite many difficulties with his model,
Bohr successfully predicted the existence and properties of element 72,
Hafnium, which he later named after his home city, Copenhagen (Havn).

Best Wishes,
Jim Diamond, chair        Chemistry Department  Linfield College
jimd@calvin.linfield.edu  McMinnville OR 97128  (503)-434-2471