Atomic structure nucleus discovery

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. 

Mass spectrometry is more than 100 years old and has yielded basic results and profound insights for the development of atomic physics. The rapid development of nuclear physics, in particular, would be unthinkable without the application of mass spectrometric methods. Mass spectrometry has contributed to conclusive evidence for the hypothesis of the atomic structure of matter. So far mass spectrometry has supplied specific results on the structure of the nucleus of atoms. Nobel prizes have been awarded to a number of scientists (Thomson, Wien, Aston, Paul, Fenn and Tanaka) associated with the birth and development of mass spectrometry, or in which mass spectrometry has aided an important discovery (e.g., for the discovery of fullerenes by Curl, Kroto and Smalley). 

Models of nuclei have grown in sophistication as new discoveries about subatomic particles have been made. One of the simplest was suggested by Niels Bohr, the Danish scientist who contributed a great deal to our understanding of atomic structure. Bohr compared the nucleus to a drop of liquid. His liquid drop model proposes that nucleons are packed together like the molecules in a liquid. Nucleons at the surface of the... 

Several major discoveries at the turn of the 20 century ied to our current model of atomic structure. Cathode rays were shown to consist of negative particles (electrons) that exist in ail matter. J. J. Thomson measured their mass/charge ratio and con-ciuded that they are much smalier and iighter than atoms. Robert Miliikan determined the charge of the electron, which he combined with other data to calculate its mass. Ernest Rutherford proposed that atoms consist of a tiny, massive, positive nucleus surrounded by electrons. 

In 1911, the British physicist and Nobel laureate Ernest Rutherford (1871-1937) published the article The Scattering of Alpha and Beta Particles by Matter and the Structure of the Atom in Philosophical Magazine. In this article, Rutherford reported the results of an experiment that demonstrated that the protons and electrons in atoms are not distributed homogeneously. Instead, the protons are concentrated in a relatively tiny region Rutherford called the nucleus (from the Latin, meaning kernel ). The electrons are extranuclear electrons are located in a relatively much larger volume of space surrounding the nucleus. Rutherford s discovery of the nucleus was immediately accepted within the scientific community. However, the relationship, if any, between atomic structure and properties was still unclear. 

Discovery of the Atom s Nucieus Rutherford s gold foil experiment probed atomic structure, and his results led to the nuclear model of fhe atom, which, with minor modifications to accommodate neutrons, is still valid today. In this model, the atom is composed of protons and neutrons—which compose most of the atom s mass and are grouped together in a dense nucleus—and electrons, which compose most of the atom s volume. Protons and neutrons have similar masses (1 amu), while electrons have a much smaller mass (0.00055 amu). Discovery of the Atom s Nucleus We can understand why this is relevant by asking, what if it were otherwise What if matter were not mostly empty space While we cannot know for certain, it seems probable that such matter would not form the diversity of substances required for life—and then, of course, we would not be around to ask the question. 

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. 

FIGURE 1.4 Ernest Rutherford (1871-1 137), who was responsible for many discoveries about the structure of the atom and its nucleus. 

The discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

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. 

From 50 years to 100 years after Dalton proposed his theory, various discoveries showed that the atom is not indivisible, but really is composed of parts. Natural radioactivity and the interaction of electricity with matter are two different types of evidence for this subatomic structure. The most important subatomic particles are listed in Table 3-2, along with their most important properties. The protons and neutrons occur in a very tiny nucleus (plural, nuclei). The electrons occur outside the nucleus. 

Marie Sklodowska Curie (1867—1934) and Pierre Curie (1859—1906) are credited with discovering polonium as they sought the source of radiation in pitchblende after they removed the uranium from its ore. Their discovery in 1898 led to the modern concepts of the nucleus of the atom, its structure, and how it reacts. 

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. 

Dalton s atoms were not perceived as possessing structure, but the discovery of electrons, and the distinctive phenomenon of radioactivity, inevitably generated interest in the way atoms were put together. Clearly, this had a bearing on the periodic table as any proposed structure must explain atomic weights.24"28 The Aufbau Principle, that each element possessed one more proton in the nucleus and one more electron in the outer shell than the preceding element, effectively systematized the periodic table29-30... 

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. 

NMR spectroscopy is probably the most powerful research tool used in structural chemical investigations today. It was first described independently by Felix Bloch (paraffin, 30 MFIz) and Edward Mills Purcell (water, 8 MFIz) in 1945/1946. They shared the Nobel Prize in Physics in 1952 for their discovery. Atomic nuclei experience this phenomenon if they possess a spin the nuclei of some elements possess a net magnetic moment (ji), viz. in the case that the spin (T) of the nucleus is non-zero. This condition is met if the mass number and the atom number of the nucleus are not both even (as is the case for 12C and 160). 

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