Americium electronic structure

As the atomic number increases, the radial extension and the bandwidth of the 5/ electrons decreases. From americium on the 5/ electrons are localized, nonbonding, and carry a magnetic moment. The actinide metals americium to californium and lawrencium are trivalent metals. Einsteinium to nobelium are divalent metals due to very high promotion energies needed to promote one / electron to the metallic bonding state as known from ytterbium in the lanthanide series. Thus, the actinide series displays more complex electronic structures than does the lanthanide series not only in the first half of thek series. 

The higher actinide metals americium, curium, berkelium and californium have - at normal pressure - again the common structure dhcp and are in this respect similar to some of the lanthanide metals. In fact, the theoretical calculations and certain experimental observations show that in these actinide metals, 5 f electrons are localized, as are the 4f electrons in the lanthanide metals. More detailed considerations on the possible correlations between electronic and crystal structure are found in. ... 

For elements with localized 5f-electrons (Am to Cf), the symmetric dhcp metal structme resembles that of the light lanthanides. However, high pressure relieves the f-f overlap and the americium structure becomes the same as uranium. 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

The orbital polarization scheme has been applied to several systems (see references above) where it improved the agreement between theory and experiment. A recent application to americium was reported by Soderlind et al.,[110] who examined structural changes of Am under pressure. The results were consistent with a high-pressure phase with delocalized 5/ electrons and a low-pressure phase with localized and non-bonding 5/ states, a Mott transition. 

Protons and neutrons make up the central part of the nucleus of the atom their internal structure is not relevant here. The electrons take up orbits around the nucleus and, in an electrically neutral atom, the number of electrons equals the number of protons. The element itself is defined by the number of protons in the nucleus. For a given element, however, the number of neutrons can vary to form different isotopes of that element. A particular isotope of an element is referred to as a nuclide. A nuclide is identified by the name of the element and its mass, for example, carbon-14. There are 90 naturally-occurring elements additional elements, such as plutonium and americium, have been created by man, for example, in nuclear reactors. 

In proceeding across the actinide series, two major and concomitant events occur (1) more 5f electrons are added, and (2) the 5f bands narrow. As 5f electrons are added, they remain in the valence band and hybridize strongly with the s-d electrons. In contrast to their importance in the rare earths, s-d electrons contribute little to the chemistry and physics of the early actinide metals compared to the enormous influence of the 5f electrons. Narrowing of these bands as one proceeds across the series results finally in the onset of f-electron localization. This incipient localization produces a bewildering array of temperature- and structure-dependent properties at plutonium. Localization is percipitous and complete at americium, driven finally by spin polarization. 

Actinium and thorium have no / electrons and behave like transition metals with a body-centered cubic structure of thorium. Neptunium and plutonium have complex, low-symmetry, room-temperature crystal structures and exhibit multiple phase changes with increasing temperature due to their delocalized 5/ electrons. For plutonium metal, up to six crystalline modifications between room temperature and 915 K exist. The / electrons become localized for the heavier actinides. Americium, curium, berkelium, and californium all have room-temperature, double hexagonal, close-packed phases and high-temperature, face-centered cubic phases. Einsteinium, the heaviest actinide metal available in quantities sufficient for crystal structure studies on at least thin films, has a face-centered cubic structure as typical for a divalent metal. 

The differences between the actinide and lanthanide metals can be rationalized by a consideration of the differences between the 4f- and 5f-electron shells [25]. In the 4f series, all the 4f electrons (added after cerium) are buried in the interior of the electron cloud. The 4f electrons are thus confined to the core of the atom, and experience relatively little interaction with electrons in the 5d shell. The maxima in the radial charge density occur well inside the usual interatomic distances in solids, and consequently the 4f electron properties of the free atoms are retained in the metallic as well as ionic lanthanide solids. Cerium is the only 4f metal that does not conform to this generalization, presumably because its 4f-electron shell is not yet fully stabilized. The actinide 5f electrons behave quite differently. For the early members of the actinide series, the Sf electrons have a greater radial distribution than do their 4f homologs. The first few 5f electrons are not confined to the core of the atom, and they can therefore interact or mix with the other valence electrons to affect interatomic interactions in the solid state. Beyond plutonium, all the 5f electrons are localized within the atomic core, and the resemblance between the f-block elements becomes closer. Americium is the first actinide metal whose crystal structure resembles that of the lanthanide metals. In the transcurium metals, the resemblance to the lanthanide metals becomes increasingly stronger. The room-temperature crystal structure for the elements for Am to Cf is dhep, just as it is in the light lanthanides. 

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