Nuclear structure of the atom

Atoms with the same value of Zbut different values of A are isotopes (Table 11.1). Many isotopes are stable but others are naturally or artificially radioactive, i.e. their atomic nuclei disintegrate, emitting particles or radiation. This changes the nuclear structure of the atom and often results in the production of a different element. 

Beginning with J.J. Thomson s discovery of the electron in 1897, developments came quickly. In 1911, Ernest Rutherford proposed the nuclear structure of the atom, and by 1920 he had named the proton and the neutron. All of this work was made possible by the discovery of X-rays in 1895, which allowed physicists to probe the atom, and by the discovery of radioactivity in 1896. The phenomenon of radioactivity destroyed the ancient concept of the immutability of the atom once and for all and demonstrated that one element could be transformed into another, thus in a sense achieving the goal that the alchemists had sought in vain. 

Rutherford, Ernest (1st Baron Rutherford of Nelson) (1871-1937) New Zealand-born British pioneer of subatomic particle physics who proposed the nuclear structure of the atom. He studied the radioactive disintegration of elements and correctly predicted the existence of the neutron. Rutherford won the Nobel Prize in chemistry in 1908. 

Rutherford of Nelson) proposes the nuclear structure of the atom. 

The unexplained phenomena included the observation of atomic line spectra, the nuclear structure of the atom, the nature of light, and the photoelectric effect. Certain experimental observations in these areas did not conform to the expectations of classical mechanics. But to really see why a new mechanics was necessary, it is important to review each of these phenomena and understand why classical mechanics did not explain the observations. 

Scientists have developed a highly sophisticated view of the structure of the atom. The currently accepted model is called the nuclear atom. We shall present it without trying to show immediately all of the experimental evidence that led to this particular model. Rest assured, though, that every feature of the nuclear atom picture rests upon experimental evidence, as we shall see in Chapter 14. 

This chapter builds an understanding of atomic structure in four steps. First, we review the experiments that led to our current nuclear model of the atom and see how spectroscopy reveals information about the arrangement of electrons around the nucleus. Then we describe the experiments that led to the replacement of classical mechanics by quantum mechanics, introduce some of its central features, and illustrate them by considering a very simple system. Next, we apply those ideas to the simplest atom of all, the hydrogen atom. Finally, we extend these concepts to the atoms of all the elements of the periodic table and see the origin of the periodicity of the elements. 

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. 

This experiment established the nuclear model of the atom. A key point derived from this is that the electrons circling the nucleus are in fixed stable orbits, just like the planets around the sun. Furthermore, each orbital or shell contains a fixed number of electrons additional electrons are added to the next stable orbital above that which is full. This stable orbital model is a departure from classical electromagnetic theory (which predicts unstable orbitals, in which the electrons spiral into the nucleus and are destroyed), and can only be explained by quantum theory. The fixed numbers for each orbital were determined to be two in the first level, eight in the second level, eight in the third level (but extendible to 18) and so on. Using this simple model, chemists derived the systematic structure of the Periodic Table (see Appendix 5), and began to... 

In the early part of the twentieth century, then, a simple model of atomic structure became accepted, now known as the Rutherford nuclear model of the atom, or, subsequently, the Bohr-Rutherford model. This supposed that most of the mass of the atom is concentrated in the nucleus, which consists of protons (positively charged particles) and neutrons (electrically neutral particles, of approximately the same mass). The number of protons in the nucleus is called the atomic number, which essentially defines the nature of... 

The first sections of this reference book set the stage for the presentation of the elements. First is the section How to Use This Book followed by a short introduction. Next is A Short History of Chemistry, the narrative of which progresses from prehistoric times to the Age of Alchemy and then to the Age of Modern Chemistry. Next is the section titled Atomic Structure, which traces the history of our knowledge of the structure of the atom some theoretical models, including quantum mechanics the discovery of subatomic (nuclear) particles... 

Atomic spectra, which historically contributed extensively to the development of the theory of the structure of the atom and led 10 the discovery of the electron and nuclear spin, provide a method of measuring ionization potentials, a method for rapid and sensitive qualitative and quantitative analysis, and data for the determination of the dissociation energy of a diatomic molecule. Information about the type of coupling of electron spin and orbital momenta in the atom can be obtained with an applied magnetic field. Atomic spectra may be used to obtain information about certain regions of interstellar space from the microwave frequency emission by hydrogen and to examine discharges in thermonuclear reactions. 

RUTHERFORD, ERNEST (1871-1937). Rutherford was a British physicist who was bom in the South Island of New Zealand and is famous for his pioneering work in nuclear physics and for his theory of the structure of the atom. 

Our next task is to combine the wavelike properties of electrons with the nuclear model of the atom and hope to explain the strange ladder of energy levels in a hydrogen atom observed experimentally. In doing so, we shall greatly enrich our model of the internal structure of an atom. 

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. 

From the beginning of his studies Langmuir was especially interested in the structure of the atom. The nature of the atom s structure was still very much in doubt. Many had crossed swords with nature to wrest this secret from her. Kelvin had pictured the atom as consisting of mobile electrons embedded in a sphere of positive electrification. J. J. Thomson had developed this same idea but his model, too, had failed because it could not account for many contradictory phenomena. Rutherford s nuclear theory of the atom as a solar system was also objected to as incomplete. The greatest difficulty to the acceptance of these models was that they all lacked a consistent explanation of the peculiar spectra of gaseous elements when heated to incandescence. 

When we now speak of nuclear magnetic resonance, we are discussing the kind of NMR discovered by Bloch and Purcell, that is, nuclear magnetic resonance in bulk materials. The early work in NMR was concentrated on the elucidation of the basic phenomena (much of which we cover in Chapters 2 and 8) and on the accurate determination of nuclear magnetic moments, which were of interest in elucidating aspects of the structure of the atomic nucleus. 

Under normal conditions, a chemical reaction involves the electrons occupying the outermost shells, or valence shells, of the atoms involved. Hence the chemical properties of an atom arise from its tendency to lose electrons from, or to attract electrons to, its valence shell. This tendency will depend upon the electronic structure of the atom and the nuclear charge experienced by the valence shell electrons. Thus, in order to explain the chemistry of a transition element, it is first necessary to consider its atomic structure and how this influences the binding of its valence shell electrons. 

Continuous wave operation of COIL is facilitated by the hyperfine structure of the atom. Iodine has a nuclear spin of, so the P /2 and Pz/2 levels are split by hyperfine interactions. Figure 8 shows the allowed transitions between the hyperfine sublevels and a high resolution emission spectrum. The F = 3 — F" = 4 transition is most intense, and this is the laser line under normal conditions. Collisional relaxation between the hyperfine sub-levels of Pz 2 maintains the population inversion, while transfer between the Fi/2 levels extracts energy stored in the F = 2 level. Hence, if it is not sufficiently rapid, hyperfine relaxation can limit power extraction. 

Conventional, chemical explosives get their power from the rapid rearrangement of chemical bonds, the links between atoms made by sharing electrons. In chemical explosives, atoms dissociate from other atoms and form new associations this releases energy, but the atoms themselves do not change. Nuclear weapons are based on an entirely different principle. They derive their explosive power from changes in the structure of the atom itself, specifically, in the core of the atom, its nucleus. 

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