Atomic nucleus, 112 discovery

E. P. Wigner (Princeton) the theory of the atomic nucleus and elementary particles, particularly through the discovery and application of fundamental symmetry principles. 

A. Bohr (Copenhagen), B. Mottelson (Copenhagen) and J. Rainwater (New York) discovery of the connection between collective motion and particle motion in atomic nuclei and the development of the theory of the structure of the atomic nucleus based on this connection. 

Fermi had been fascinated by the discovery of the neutron by James Chadwick in 1932. He gradually switched his research interests to the use of neutrons to produce new types of nuclear reactions, in the hope of discovering new chemical elements or new isotopes of known elements. He had seen at once that the uncharged neutron would not be repelled by the positively-charged atomic nucleus. For that reason the uncharged neutron could penetrate much closer to a nucleus without the need for high-energy particle accelerators. lie discovered that slow neutrons could... 

Chemists were not able to use their methods to determine the structure of the atom. The discovery of radioactivity by Henri Becquerel and the work of Marie and Pierre Curie showed, however, that heavy elements were not stable. The earlier postulate of their indivisibility could no longer be maintained. In 1906 Ernest Rutherford made the next horrorif-ic revelation his scattering experiments showed that the atom was almost empty. A tiny nuclear mass was circled by electrons at a large distance. For comparison, if the nucleus were the size of a cherry pit and were placed in the center of a football field, the electrons would be circulating in the back rows of the stadium. If the nucleus were the size of a football, the first electrons would be circling it at a distance of one kilometer. Between them would be absolute emptiness. 

Rutherford s atomic model solved problems inherent in Thomson s atomic model, but it also raised others. For example, an atomic nucleus composed entirely of positive charges should fly apart due to electrostatic forces of repulsion. Furthermore, Rutherford s nuclear atom could not adequately explain the total mass of an atom. The discovery of the neutron, in 1932, eventually helped to settle these questions. 

The discovery of the atomic nucleus affected all sciences, including biology and its applications in the field of medicine. During her graduate studies at the University of Western Ontario, Dr. Karen Goulenchyn excelled in theoretical mathematics, but switched to medicine because she was drawn to practical applications of science in people s lives. An interest in computers led her to nuclear medicine, which she now practices at the Civic Hospital in Ottawa. 

Rutherford s discovery of the atomic nucleus was his greatest contribution to physics and it established him as the leading experimental physicist of his day. However, it was only a beginning, and many questions about the atom remained unanswered. As yet nothing was known about electron orbits or about the relationship between the structure of the atom and the periodic table. Before Rutherford performed his experiments, it was thought that the atom was understood. Now it was apparent that much remained to be learned. But then great discoveries in physics seem always to suggest new questions and open up new lines of research. The more that is known, the better the picture scientists have of what remains unknown. 

Shortly after coming to Rutherford s laboratory, Bohr set to work on the problem of understanding the structure of atoms. Rutherford s discovery of the atomic nucleus had introduced formidable problems. It seemed necessary to assume that the electrons in an atom orbited the nucleus. Otherwise, the electrical attraction between the electrons and the nucleus would cause the electrons and the nucleus to collide with one another. But, as we have seen, the assumption that the electrons orbited the nucleus didn t seem to work either. Orbiting electrons should lose energy and fall into the nucleus anyway. 

In 1934 the Japanese physicist Hideki Yukawa postulated the existence of yet another force particle, which he called the meson. In 1932 Yukawa began his academic career with an appointment at Osaka Imperial University, which had been founded the previous year. The discovery of the neutron and the publication of Fermi s theory started him thinking about the nature of the force that bound protons and neutrons together in an atomic nucleus. He realized that, though... 

In 1899 he identified two forms of radioactivity, which he called alpha and beta particles. As we saw earlier, he deduced that alpha particles are helium nuclei. Beta particles are electrons - but, strangely, they come from the atomic nucleus, which is supposed to be composed only of protons and neutrons. Before the discovery of the neutron this led Rutherford and others to believe that the nucleus contained some protons intimately bound to electrons, which neutralized their charge. This idea became redundant when Chadwick first detected the neutron in 1932 but in fact it contains a deeper truth, because beta-particle emission is caused by the transmutation ( decay ) of a neutron into a proton and an electron. 

Element abundance data were useful not only in astrophysics and cosmology but also in the attempts to understand the structure of the atomic nucleus. [74] As mentioned, this line of reasoning was adopted by Harkins as early as 1917, of course based on a highly inadequate picture of the nucleus. It was only after 1932, with the discovery of the neutron as a nuclear component, that it was realized that not only is the atomic mass number related to isotopic abundance, but so are the proton and neutron numbers individually. Cosmochemical data played an important part in the development of the shell model, first proposed by Walter Elsasser and Kurt Guggenheimer in 1933-34 but only turned into a precise quantitative theory in the late 1940s. [75] Guggenheimer, a physical chemist, used isotopic abundance data as evidence of closed nuclear shells with nucleon numbers 50 and 82. 

The chemical shift is the cornerstone of the chemical applications of NMR. As we noted in Chapter 1, this accidental discovery converted a technique designed initially for probing the structure of the atomic nucleus into one that can provide detailed information about the structure and dynamics of molecules. In this chapter we examine the theoretical underpinning of the chemical shift, explore empirical correlations between chemical shifts and functional groups in organic molecules, and describe simple physical models that can help us to understand and predict chemical shifts. 

The first hint of the actual raw material out of which atoms are constructed came with the discovery of the electron by J. J. Thomson in 1897. Then Ernest Rutherford, fourteen years later, discovered the atomic nucleus. After Rutherford s 1911 finding, the idea that atoms were made of negatively charged electrons and a positively charged massive core quickly gained widespread acceptance, and further discoveries followed rapidly. But first, we must back up a few years to pick up another strand of history. 

Once again, the hydrogen atom was a source of inspiration— this time for a chemist. The discovery of deuterium by Harold Urey has guided the thinking and experiments of physicists as they have sought to expose the forces at play inside the atomic nucleus. 

Following James Chadwick s discovery of the neutron in 1932, and throughout the decade of the 1930s, the atomic nucleus became the frontier of physical research. And pushing the boundaries of this frontier were many American physicists. 

This discovery required physicists to abandon the assumption of central forces acting within the atomic nucleus. Central forces were well understood and easily handled theoretically thus, physicists were reluctant to abandon the idea of central forces. On November 28, 1939, J. H. Van Vleck from Harvard University wrote to Rabi, I am not at all clear on just exactly the details of the setup by which you are deducing this celebrated quadrupole moment.. .. If you can send me the details of just what you are doing, I can assure you I will do my best to punch holes in the attempted deduction of a quadrupole moment. One week later, on December 5, Van Vleck wrote again to Rabi, I have thought quite a bit about your experiments, but I cannot find any loop holes.. .. I congratulate you on the most interesting results of your experiments. ... 

The Discovery of the Atomic Nucleus. In 1911 Ernest Rutherford (1871-1937), then Professor of Physics at the University of Manchester, England, carried out some experiments which showed that most of the mass of atoms is concentrated in particles which are very small in size compared with the atoms themselves. The method that he used is very simple, and the arguments involved in drawing conclusions from them are also very simple. 

With the discovery of the neutron by Chadwick in 1932, the structure of the atomic nucleus was clarified. A nucleus of atomic number Z and mass number A was composed of Z protons and A — Z neutrons. Nuclear diameters arc of the order of several times 10 m. From the iiers ieelive of an atom, which is 10 times larger, a nucleus behaves, for most iiurposes, like a point charge +Ze. 

The discovery of this interaction led to the postulation that an atomic nucleus possesses a spin-angular momentum represented by the spin angular momentum vector h, where I is the nuclear spin and h is Planck s constant, h, divided by Itt. It has been found experimentally that I is an odd integer multiple of for nuclei of odd atomic mass numbers (isotope number), zero for nuclei of even atomic mass numbers and even nuclear charges (atomic number), and an integer for nuclei of even atomic mass numbers and odd nuclear charges. The nuclei that we are concerned with here, H, C, and F, have an I of 5. 

Ernest Rutherford announces the discovery of the atomic nucleus. 

Figure 2.6 Rutherford s a-scattering experiment and discovery of the atomic nucleus. A, HYPOTHESIS Atoms consist of electrons embedded in diffuse, positively charged matter, so the speeding a particles should passthrough the gold foil with, at most, minor deflections.

With the discovery of the neutron as a fundamental particle, many paradoxes of physics and chemistry were finally resolved, and new areas of research evolved. Prior to the discovery of the neutron as a fundamental particle, scientists generally believed that the nucleus was comprised of protons and nuclear electrons. However, one could not explain, for example, the spin of nuclei with that model. Now, at last, theory could predict the properties of the nucleus quite well. Also, since neutrons are not repelled by the charge on the atomic nucleus, they interact easily with nnclei. Nen-tron scattering enables the determination of crystal stmctnres by probing the positions of nuclei in a sample. Neutrons can also catalyze fission reactions, for example, the fission of uranium nuclei that led to the creation of nuclear power plants and the atomic bomb. 

While I am not a licensed lit-crit (literary critic), I think that a 1916 reviewer of another Lewis book had him properly pegged as a novelist The plot moves swiftly with the help of incredible coincidences and improbable ro-mances. Nevertheless, let us give the author very considerable credit as a knowledgeable and sophisticated observer of chemistry. He was amazingly well informed and current about the complex revolution in the understanding of the structure of the atomic nucleus that was very much in motion as he wrote White Lightning. Marvin reads of Henry G.J. Moseley s discovery of atomic numbers in 1914 12 unknown Moseley had found it—a sure way to determine the... 

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