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1912-01-01
The Bohr atom
Upon completing his doctorate in physics at the University of Copenhagen in 1911, Bohr obtained a grant from the Carlsberg Foundation to study abroad. He went first to Cambridge to study with J.J. Thomson, known as the discoverer of the electron, the theory of which had been the topic of Bohr's doctoral thesis.
After half a year he was invited by Ernest Rutherford in Manchester to continue his studies there. Only a year before, Rutherford and his collaborators had established that the atom consisted of a positively charged central nucleus carrying practically the entire mass, with negatively charged electrons circling around it at relatively large distances.
This would become the basis for Bohr's breakthrough as a physicist, as he realised that such a system would be unstable according to classical physics and that a radical solution was required.
He employed the quantum of action, introduced by Max Planck in 1900, postulating in complete contradiction to classical physics that the electrons could circle only in a limited number of discrete orbits, releasing or absorbing electromagnetic radiation when jumping from one orbit to the other.
Back in Copenhagen, Bohr was alerted by his colleague H.M. Hansen to the Balmer formula of experimental spectroscopy, an empirically derived formula which described, but did not explain, the spectrum of the hydrogen atom. It turned out that Bohr's theory predicted exactly this formula.
His famous ‘trilogy' of papers presenting his theory followed in 1913. In the course of the next decade, Bohr's theory was improved and modified by Bohr and others. Predicting a great number of experimental findings, it was gradually accepted in the physics community.
(Written by Finn Aaserud, director, Niels Bohr Archive)