Bohr, Niels Henrik David

 

Niels Henrik David Bohr was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in Copenhagen. He was also part of the team of physicists working on the Manhattan Project.

In 1911, under Sir J.J. Thomson's guidance Bohr assisted in the experimental work going on in the Cavendish Laboratory, and at the same time he continued to pursue his own theoretical studies. In the spring of 1912 he went to work at Professor Rutherford's laboratory in Manchester, where he preformed theoretical work on the absorption of alpha rays.

In 1913, Bohr published a theory about the structure of the atom based on an earlier theory of Rutherford's. Rutherford had shown that the atom consisted of a positively charged nucleus, with negatively charged electrons in orbit around it. Bohr expanded upon this theory by proposing that electrons travel only in certain successively larger orbits. He suggested that the outer orbits could hold more electrons than the inner ones, and that these outer orbits determine the atom's chemical properties.

Bohr also proposed adding to the model the new idea of quanta put forth by Max Planck in 1901. That way, electrons existed at set levels of energy, that is, at fixed distances from the nucleus. If the atom absorbed energy, the electron jumped to a level further from the nucleus; if it radiated energy, it fell to a level closer to the nucleus. His model was a huge leap forward in making theory fit the experimental evidence that other physicists had found over the years. Later other physicists expanded his theory into quantum mechanics. This theory explains the structure and actions of complex atoms.

Bohr's later activities in his Institute were directed toward research on the atomic nuclei, and of their transmutations and disintegrations. In 1936 he pointed out that the transition processes in atomic nuclei could be described in a classical unlike the way that an atom is described. This was in part due to the size of the transition, compared to the vastness of the region that it occurred in. According to this view, a liquid drop would give a very good picture of the nucleus; this was to become the “liquid droplet theory”.  In 1939 Hahn, Meitner and Strassmann used this theory to help further the understanding of nuclear fission when they were working on splitting uranium.

Bohr was involved in a long-running series of exchanges with Einstein about the foundations and implications of quantum theory, conversations that became known as the "Bohr-Einstein debate."

During the Nazi occupation of Denmark in Wotrld War II, Bohr escaped to Sweden and spent the last two years of the war in England and America, working on the Manhatan Project. Bohr believed that atomic secrets should be shared by the international scientific community. After meeting with Bohr, J. Robert Oppenheimer suggested Bohr visit President Franklin D. Roosevelt to convince him that the Manhattan Project should be shared with the Russians in the hope of speeding up its results. Roosevelt suggested Bohr return to the United Kingdom to try to win British approval. Winston Churchill disagreed with the idea of openness towards the Russians to the point that he wrote in a letter: "It seems to me Bohr ought to be confined or at any rate made to see that he is very near the edge of mortal crimes." Bohr's views on this subject are especially set forth in his Open Letter to the United Nations, June 9, 1950.

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