Heaviest elements

Symbol U atomic number 92 atomic weight 238.029 an actinide series radioactive element heaviest naturally-occurring element electron configuration J Rn]5/36(ii7s2 valence states +2, -i-3, +4, -i-5, -1-6 ionic radii 1J3+ l.OSA, IJ4+ O.89A, 0.76A, for coordination number 6 and U 0.45 A and 0.81 A... 

Crude oils form a continuum of chemical species from gas to the heaviest components made up of asphaltenes it is evidently out of the question, given the complexity of the mixtures, to analyze them completely. In this chapter we will introduce the techniques of fractionation used in the characterization of petroieum as well as the techniques of elemental analysis applied to the fractions obtained. 

Before it was known that elements beyond uranium were capable of existence, the heaviest known natural elements, thorium, protactinium and uranium, were placed in a sixth period of the periodic classification, corresponding to the elements hafnium, tantalum and tungsten in the preceding period. It was therefore implied that these elements were the beginning of a new, fourth transition series, with filling of the penultimate n = 6 level (just as the penultimate = 5... 

Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN. The specific gravity of iridium is only very slightly lower than osmium, which is generally credited as the heaviest known element. Calculations of the densities of iridium and osmium from the space lattices give values of 22.65 and 22.61 g/cm 3, respectively. These values may be more reliable than actual physical measurements. At present, therefore, we know that either iridium or osmium is the densest known element, but the data do not yet allow selection between the two. 

From radium called niton at first, L. nitens, shining) The element was discovered in 1900 by Dorn, who called it radium emanation. In 1908 Ramsay and Gray, who named it niton, isolated the element and determined its density, finding it to be the heaviest known gas. It is essentially inert and occupies the last place in the zero group of gases in the Periodic Table. Since 1923, it has been called radon. 

November 9,1994 at 4 39 pm, the first atom of the heaviest chemical atom with atomic number 110 was detected at the Gesellschaft fur Schwerionenforschung (GSI) in Darmstadt, in Germany. For the last ten years, this element has been the subject of an intense search by many laboratories world-wide. 

February 9,1996 at 10 37 pm, at the Gesellschaft fur Schwerionenforschung in Darmstadt, Germany a team of scientists discovered their sixth element. This element has the atomic number 112 and is currently the heaviest element ever produced by man. It has an atomic mass of 277. 

For each element, the number of protons is fixed. Thus, for hydrogen (Z = 1) there is just one proton (P = 1) for the next element, helium (Z = 2), there are just two protons (P = 2) and so on up to the heaviest natural element, uranium, which has atomic number 92 and therefore has Z = P = 92. 

Special techniques for experimentation with the actinide elements other than Th and U have been devised because of the potential health ha2ard to the experimenter and the small amounts available (15). In addition, iavestigations are frequently carried out with the substance present ia very low coaceatratioa as a radioactive tracer. Such procedures coatiaue to be used to some exteat with the heaviest actinide elements, where only a few score atoms may be available they were used ia the earHest work for all the transuranium elements. Tracer studies offer a method for obtaining knowledge of oxidation states, formation of complex ions, and the solubiHty of various compounds. These techniques are not appHcable to crystallography, metallurgy, and spectroscopic studies. 

Comphcated theoretical calculations, based on filled shell (magic number) and other nuclear stabiUty considerations, have led to extrapolations to the far transuranium region (2,26,27). These suggest the existence of closed nucleon shells at Z = 114 (proton number) and N = 184 (neutron number) that exhibit great resistance to decay by spontaneous fission, the main cause of instabiUty for the heaviest elements. Eadier considerations had suggested a closed shell at Z = 126, by analogy to the known shell at = 126, but this is not now considered to be important. 

The effects of a rather distinct deformed shell at = 152 were clearly seen as early as 1954 in the alpha-decay energies of isotopes of californium, einsteinium, and fermium. In fact, a number of authors have suggested that the entire transuranium region is stabilized by shell effects with an influence that increases markedly with atomic number. Thus the effects of shell substmcture lead to an increase in spontaneous fission half-Hves of up to about 15 orders of magnitude for the heavy transuranium elements, the heaviest of which would otherwise have half-Hves of the order of those for a compound nucleus (lO " s or less) and not of milliseconds or longer, as found experimentally. This gives hope for the synthesis and identification of several elements beyond the present heaviest (element 109) and suggest that the peninsula of nuclei with measurable half-Hves may extend up to the island of stabiHty at Z = 114 andA = 184. 

Radon is the heaviest of the hehum-group elements and the heaviest of the normal gaseous elements. It is strongly radioactive. The most common isotope, Rn, has a half-life of 3.825 days (49). Radon s scarcity and radioactivity have severely limited the examination of its physical properties, and the values given ki Table 3 are much more uncertain than are the values Hsted for the other elements. 

Soil Nutrient. Molybdenum has been widely used to increase crop productivity in many soils woddwide (see Fertilizers). It is the heaviest element needed for plant productivity and stimulates both nitrogen fixation and nitrate reduction (51,52). The effects are particularly significant in leguminous crops, where symbiotic bacteria responsible for nitrogen fixation provide the principal nitrogen input to the plant. Molybdenum deficiency is usually more prominent in acidic soils, where Mo(VI) is less soluble and more easily reduced to insoluble, and hence unavailable, forms. Above pH 7, the soluble anionic, and hence available, molybdate ion is the principal species. 

Tantalum [7440-25-7] atomic number 73, is the heaviest element in Group 5 (VA) of the Periodic Table. This tough, ductile, silvery gray metal has an atomic weight of 180.948 amu. The element was discovered by A. K. Ekeberg in 1802 in minerals taken from Kimito, Finland, and Ytterby, Sweden (1). 

Considering that heavy elements have more levels than just K and L, Eq. (2.2) also indicates that the heavier the element, the more numerous are the possible Auger transitions. Fortunately, there are large differences between the probabilities of different Auger transitions, so that even for the heaviest elements, only a few intense transitions occur, and analysis is still possible. 

Radon A radioactive element, the heaviest of the noble gases, formed by the radioactive decay of radium. 

The isolation and identification of 4 radioactive elements in minute amounts took place at the turn of the century, and in each case the insight provided by the periodic classification into the predicted chemical properties of these elements proved invaluable. Marie Curie identified polonium in 1898 and, later in the same year working with Pierre Curie, isolated radium. Actinium followed in 1899 (A. Debierne) and the heaviest noble gas, radon, in 1900 (F. E. Dorn). Details will be found in later chapters which also recount the discoveries made in the present century of protactinium (O. Hahn and Lise Meitner, 1917), hafnium (D. Coster and G. von Hevesey, 1923), rhenium (W. Noddack, Ida Tacke and O. Berg, 1925), technetium (C. Perrier and E. Segre, 1937), francium (Marguerite Percy, 1939) and promethium (J. A. Marinsky, L. E. Glendenin and C. D. Coryell, 1945). 

It will be recalled that is 100% abundant and is the heaviest stable nuclide of any element (p. 550), but it is essential to use very high purity Bi to prevent unwanted nuclear side-reactions which would contaminate the product Po in particular Sc, Ag, As, Sb and Te must be <0.1 ppm and Fe <10ppm. Polonium can be obtained directly in milligram amounts by fractional vacuum distillation from the metallic bismuth. Alternatively, it can be deposited spontaneously by electrochemical replacement onto the surface of a less electropositive metal... 

Six isotopes of element 106 are now known (see Table 31.8) of which the most recent has a half-life in the range 10-30 s, encouraging the hope that some chemistry of this fugitive species might someday be revealed. This heaviest isotope was synthsised by the reaction Cm( Ne,4n) 106 and the present uncertainty in the half-life is due to the very few atoms which have so far been observed. Indeed, one of the fascinating aspects of work in this area is the development of philosophical and mathematical techniques to define and deal with the statistics of a small number of random events or even of a single event. 

Uranium (symbol U atomic number 92) is the heaviest element to occur naturally on Earth. The most commonly occurring natural isotope of uranium, U-238, accounts for approximately 99.3 percent of the world s uranium. The isotope U-235, the second most abundant naturally occurring isotope, accounts for another 0.7 percent. A third isotope, U-234, also occurs uatiirally, but accounts for less than 0.01 percent of the total naturally occurring uranium. The isotope U-234 is actually a product of radioactive decay of U-238. 

Fig 1-17 K, L, and M x ray energy-level diagram for a heavy element (uranium). The heaviest lines are those of major analytical interest. Lines of occasional analytical interest are of medium weight. The energy of a state is that which an atom has when an electron is missing from the level corresponding to that state. 

At present, most high-voltage sources for ordinary spectrographs are designed for a 60-kv maximum, which is too low to excite the K lines of the heavier elements. Sources capable of exciting the K lines of even the heaviest elements can be built, but until recently, only at a considerable increase in cost. For the ordinary laboratory, it has been more economical to use the L lines of the heavier elements, which can be excited below 60 kv, than to invest in a 100-kv source. 

Technological advances in electronics early in the twentieth century led to the invention of the mass spectrometer, a device for determining the mass of an atom (Fig. B.5). Mass spectrometers are described more fully in Major Technique 6 after Chapter 18. Mass spectrometry has been used to determine the masses of the atoms of all the elements. We now know, for example, that the mass of a hydrogen atom is 1.67 X 10 27 kg and that of a carbon atom is 1.99 X 10 26 kg. Even the heaviest atoms have masses of only about 5 X 10-25 kg. Once we know the mass of an individual atom, we can determine the number of those atoms in a given mass of element simply by dividing the mass of the sample by the mass of one atom. 

Iridium, the heaviest element of the cobalt group, was found to display the least tendency towards cluster formation at 10-12 K (49), which, as already mentioned, was quite facile for Co and Rh (49). Considering the plethora of sharp, well-defined, atomic-resonance lines observed for Ir (see Fig. 6) compared to those of Co and Rh, the remarkably impressive correlation with the representation of the gas-... 

During the red giant phase of stellar evolution, free neutrons are generated by reactions such as C(a,n) and Ne(a,n) Mg. (The (ot,n) notation signifies a nuclear reaction where an alpha particle combines with the first nucleus and a neutron is ejected to form the second nucleus.) The neutrons, having no charge, can interact with nuclei of any mass at the existing temperatures and can in principle build up the elements to Bi, the heaviest stable element. The steady source of neutrons in the interiors of stable, evolved stars produces what is known as the "s process," the buildup of heavy elements by the slow interaction with a low flux of neutrons. The more rapid "r process" occurs in... 

Because the path of the s process is blocked by isotopes that undergo rapid beta decay, it cannot produce neutron-rich isotopes or elements beyond Bi, the heaviest stable element. These elements can be created by the r process, which is believed to occur in cataclysmic stellar explosions such as supemovae. In the r process the neutron flux is so high that the interaction hme between nuclei and neutrons is shorter that the beta decay lifetime of the isotopes of interest. The s process chain stops at the first unstable isotope of an element because there is time for the isotope to decay, forming a new element. In the r process, the reaction rate with neutrons is shorter than beta decay times and very neutron-rich and highly unstable isotopes are created that ultimately beta decay to form stable elements. The paths of the r process are shown in Fig. 2-3. The r process can produce neutron-rich isotopes such as Xe and Xe that cannot be reached in the s process chain (Fig. 2-3). 

The periodic table lists all the known elements in numerical order, starting with the lightest (hydrogen) and proceeding to the heaviest (uranium, among naturally occurring elements). The list is broken into seven rows. 

For an atom to be neutral, the number of electrons that it contains must equal the total positive charge on its nucleus. Because each element has a characteristic positive charge associated with its nucleus, ranging from +1 for hydrogen to greater than +100 for the heaviest elements, atoms of different elements have different numbers of electrons. 

C22-0100. The heaviest transuranium elements are formed by bombardment with relatively heavy nuclides such as 48... 

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