In this installment of the history of atom theory, physics professor (and my dad) Dean Zollman discusses the problems physicists had with Bohr’s model – and they could get downright catty. – Kim
By Dean Zollman
If you look at a description of the Bohr model of the atom in most textbooks or many popularized accounts, you could easily get the impression that it was an immediate success. Physicists were troubled by some of its assumptions, but it worked, so they moved on. However, like several other developments that I have discussed in these posts, the history is more complex. Many physicists were not just unhappy with some of the assumptions, they were sure the entire model was absolutely wrong.
Basically the Bohr model of the atom was an attempt to use the classical physics of Newton, Maxwell, and others but incorporate the quantum ideas of Planck and Einstein. Thus, it was the beginning of a revolution but not as great as the revolution that was to come soon. As a result of Bohr’s quantum-classical blended model, five major concerns arose.
- An accelerating charged particle will emit continuous radiation. An electron moving in a circle is accelerating. Yet in Bohr’s model the orbiting electrons did not radiate. If it did, the electron would spiral quickly into the nucleus. (I discussed this one in a previous post.) Bohr had no logical reason for the nonradiating electron, but without it, atoms would not exist.
- In the early 20th century, electromagnetic radiation such as radio waves was created by shaking electrons back and forth. The frequency of the shaking determined the frequency of the wave. For radio, microwaves, etc., we still do that. A radio station that broadcasts at 99.5 megahertz shakes electrons in its antenna 99,500,000 times per second. Scientists in 1913 expected that light was emitted in the same way, so an electron in an atom would vibrate to emit light. However, in the Bohr model nothing vibrated; the light just appeared as the electron changed orbits.
- Even worse, these “quantum jumps” in orbit seemed to occur without a cause. Once an electron was in orbit with an energy higher than the lowest possible energy, it would at some time move to a lower energy, but Bohr could not ascribe a cause to that event.
- Then, physicists asked: how does the electron “know” where it is going? When it leaves one orbit, it gives off a photon of light. The energy of that photon is determined by the energy that the electron will have when it lands in the lower orbit. It seems that the electron knows what will happen to it before the event is finished.
- And, finally, what happens to the electron in between the time it leaves one orbit and the time it gets to the second orbit?
So, there were some rather good reasons to wonder if Bohr was making up a story that just did not fit with the known laws of physics. However, for many people something was more important than these concerns. The Bohr model worked in a way no other model of the atom had. Bohr had been able to reproduce the equation for the spectral lines that had been discovered by Balmer. He had also explained several recently observed effects. That was enough for some physicists, but not for all.
Arnold Sommerfeld (1888-1951) at Ludwig Maximilian University in Munich did see the positive side of Bohr’s model. He and his colleagues made modifications to the model by assuming that the electron would move in elliptical orbits rather than circles. (Then, the electrons are moving in a manner similar to the planets around the sun.)
This change made the connection between the model and recent accurate measurements even better than the model with circular orbits. Thus. These refinements to create the Bohr-Sommerfeld model gave the model more credibility than it had before.
The Bohr-Sommerfeld model of the atom (by Pieter Kuiper, public domain via Wikimedia Commons)
However, not everyone was happy with this turn of events. Paul Ehrenfest (1880-1933) wrote to Sommerfeld to congratulate him on this work. His statement was somewhat a back-handed compliment. He said, “Even I consider it horrible that this success will help the preliminary, but still completely monstrous, Bohr model on to new triumphs, I nevertheless heartily wish physics at Munich further success along this path!” Earlier Ehrenfest had written to Hendrik Lorentz (1853-1928), “Bohr’s work on the quantum theory of the Balmer formula, has driven me to despair. If this is the way to reach the goal, I must give up doing physics.” Clearly, Ehrenfest was not a fan of the Bohr model.
One of the least diplomatic critics of Bohr was Johannes Stark (1874-1957). In 1919, Stark received the Noble Prize for two discoveries. One of these discoveries, which bears his name, is that spectral lines will split into more than one line when the atoms that emit them are in an intense electric field. Other physicists were able to use the Bohr-Sommerfeld model of the atom to explain why this splitting occurs. That did not, however, stop Stark from taking an intellectual shot at Bohr.
During Stark’s Nobel Lecture, he stated Planck’s hypothesis “forms the starting point of Bohr’s theory of the emission of serial lines. Although I myself once stood on the threshold of this theory, and although the final formulae give a series of frequency relationships in the spectral series which agree well with observed facts, I am nevertheless unable to believe it, because in its provisions it postulates suppositions which contradict, not only Maxwell’s theory, but the very spirit of physics. This criticism is directed not at Planck’s quantum of action, but at the hypotheses of Bohr which are bound up with it.”
In other words, Planck’s ideas are OK, but Bohr misused them and must be wrong. I have not found any record of Stark’s thoughts when Bohr was the recipient of the Nobel Prize three years later.
Stark’s choices in politics were also not very wise. With Philipp Lenard (1862-1947) he led the Deutsche Physik (German physics, sometimes called Aryan Physics) movement during the Nazi period in Germany.
There were also many critics of Bohr’s model in England, including J.J. Thompson (1856-1940), the discoverer of the electron, and John William Nicholson (1881-1955), who developed his own atomic model.
All of the criticisms were eventually silenced when Bohr’s model was replaced by modern quantum theory. We will get to that in a couple of months. But, first we will look at an explanation for why the electrons do not radiate while moving in an orbit. This hypothesis partially helped give a foundation to one of Bohr’s assumptions and laid more of the groundwork for quantum theory. It even involves a French prince.
(Several of the quotations in this post are reproduced from a series of papers written or co-written by Helge Kragh.)
Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.