Early Atomic Discoveries

Increasing knowledge   knowledge of the atom and their emitted radiations ultimately is what lead to the development of medical applications. Ernest Rutherford, now known as the Father of Nuclear Physics, contributed much with his discovery of radioactive half-life and his identification and naming of the different alpha, beta and gamma radiations, for which he was awarded the Nobel Prize in Chemistry in 1908. His view of the atom was that of a positively charged nucleus surrounded by electrons, particles known since J.J. Thompson’s 1897 discovery (for which he received the Noble prize in Physics in 1906 ) .

Niels Bohr talked about the correspondence principle in one of the Silliman Lectures he gave at Yale University between 6 and 13 November 1923. From the correspondence principle, he derived the selection rule that for radiative transitions in atoms, the change in the azimuthal quantum number k must be ±1. Bohr used that result in establishing his electron structures for the elements. (Courtesy of the Niels Bohr Archive, Copenhagen.)
Niels Bohr talked about the correspondence principle in one of the Silliman Lectures he gave at Yale University between 6 and 13 November 1923. From the correspondence principle, he derived the selection rule that for radiative transitions in atoms, the change in the azimuthal quantum number k must be ±1. Bohr used that result in establishing his electron structures for the elements. (Courtesy of the Niels Bohr Archive, Copenhagen.)

Rutherford’s view of the atom attracted the attention of Niels Bohr, a Danish Physicist, who subsequently went to  work in Rutherford’s lab in McGill, Canada in 1912. The American Institute of Physics web page chronicles much of the story of this important period. How Bohr attempts to connect Physics and molecular properties to predict chemical reactions is described in Niels Bohr between physics and chemistry found on physics today website.  March 2013 Physics Today. An excerpt from that article is given  below:

Max Delbruck - Wikipedia
Max Ludwig Henning Delbrück, (September 4, 1906 – March 9, 1981), a German–American biophysicist, helped launch the molecular biology research program in the late 1930s. He stimulated physical scientists’ interest into biology, especially as to basic research to physically explain genes, mysterious at the time. Formed in 1945 and led by Delbrück along with Salvador Luria and Alfred Hershey, the Phage Group made substantial headway unraveling important aspects of cell physiology. The three shared the 1969 Nobel Prize in Physiology or Medicine “for their discoveries concerning the replication mechanism and the genetic structure of viruses”.

“Starting with Rutherford, his contacts/stimulations include Hevesy, Gamow, Delbruck (through his Lancet article on Light), and many of the famous people responsible for the development of Quantum mechanics and Quantum chemistry). It was Rutherford that conceived of the orbital distribution of electrons which accounted for the different wave length emissions for elements of different atomic number. Bohr then added many details regarding atomic structure leading to and complementing the development of quantum mechanics. It is for this contribution that he received the Nobel Prize in Physics in 1922. In the revised atomic model, Bohr placed the nucleus at the center of the atom with electrons in well-defined orbits around it, just as planets orbit the sun.The Bohr atom caught the attention of Max Planck, Heisenberg, Pauli and other luminaries in physics who worked to fit ideas on quantal nature of emissions, with quantum mechanical theories. Bohr established the Complementarity principle that required that in the limit, microscopic and macroscopic predictions and observations would agree. This included the many distinguished physicists clustered around the Goettingen Institute in Germany (Einstein, Born, Planck,Pauli,Weiskopf, Oppenheimer, Gamow, Delbruck and others). Many visits with the Bohr Lab, led to important spin offs to biological sciences. Bohr’s father was a physiologist, and Bohr’s inquiring mind looked for physical models for explanations of biological processes. His writings and speculations attracted the attention of several important physicists who diverted their careers to biology. Among them, Delbruck and Gamov (and Szilard later on) who collectively made significant contributions to modern biology. Delbruck was a brilliant theoretical physicist, well briefed in quantum mechanics and astrophysics. He participated in many of the Gottingen quantum mechanics discussions, but soon turned his attention to genetics working in collaboration with a Russian biologist (Timofeef-Ressovski) in Berlin. Together they made a series of observations that pointed to the gene as the elemental/fundamental structure in a 1935 publication: About the Nature of the Gene Mutation and the Gene Structure. A hit theory explanation was consistent with Muller’s 1928 observation on the effect of ionizing radiations on gene mutations”. Delbruck went on to work in the USA, Nobel prize in 1968, and is considered the Father of Molecular Biology. John Wikswo presented a chronology of his life and work at a commemorative celebration (see chronology of Delbruck by John Wikswo.

1932 is considered as  a water shed year in nuclear physics as is well chronicled in Physics today  (Moving to New Physics). It recalls nuclear physics facts before 1932: “In 1911 Ernest Rutherford discovered that atoms have a small, massive kernel, which he termed Nucleus. In 1918 Rutherford bombarded nitrogen atoms with alpha particles (helium nuclei), thus converting nitrogen to oxygen plus a liberated hydrogen nucleus. He considered the liberated hydrogen nucleus a new particle and named it the proton. It was, he conjectured, the building block of all nuclei and that it accounted for their positive charge. The existence of isotopes presented a great puzzle. Isotopes had been discovered in 1912 by Frederick Soddy in his study of the decay of uranium to radon. Soddy realized that there could be versions of an element whose atomic masses differed even though their chemical properties were the same.

The following year, J.J. Thompson succeeded in separating neon isotopes by passing a beam of neon particles through a magnetic field. In 1920 Rutherford proposed that the difference between the atomic number of a nucleus and its atomic mass might be due to neutral  particles in the nucleus. The neutral particle, he suggested, might be a combination of a proton, with a “neutral electron”. But after the 1927 publication of Werner Heisenberg’s uncertainty principle, the confinement of a putative nuclear electron with the electron’s tiny mass would impose an  implosively high kinetic energy on any nuclear electron.

Enter 1932, the year in which those problems were set on the path to a resolution. Six weeks after the discovery of deuterium, James Chadwick announced the discovery of the neutron, and in August Carl Anderson announced the discovery of a positive electron, i.e. the positron. The work of incorporating these new discoveries into the emerging understanding of nuclear physics proceeded at a breath taking pace”.   See 1932 A Water shed year in nuclear physics.

Much more information on many of the the scientists, and their interactions in the early days is well covered in the first 100 + pages of Rhodes’ book: The Making of the Atom Bomb.

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