1,721,136 research outputs found
Oxygen electronic polarizability and local atomic potentials in zeolites obtained by X-ray photoelectron spectroscopy and X-ray excited Auger spectroscopy
No full textEdited by E. Fois and A. Gamb
Un semplice metodo per introdurre il concetto di elettronegatività nel corso di chimica generale
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Comment on the paper "Surface composition and chemical bonding of thermally reduced yttria as studied by XPS and SEXAFS: Influence of Zr doping" by Thromat et al. Surf. Interface Anal. 15, 355 (1990))
No full textIn this letter we report a comment on the paper "Surface composition and chemical bonding of thermally reduced yttria as studied by XPS and SEXAFS: Influence of Zr doping" by Thromat et al. Surf. Interface Anal. 15, 355 (1990)). It is shown that the polarization energies are strongly affected by the dipole-dipole interactions between the induced dipoles around the atom with the core hole
Auger parameter shifts in the case of the non-local screening mechanism: Applications of the electrostatic model to molecules, solids and adsorbed species
No full textA simple electrostatic model is applied to predict and rationalize the Auger parameter shifts in molecules, solids and adsorbed species. The model is valid in the case of the non-local screening mechanism, i.e. when the screening of the core hole is due to the polarization of the nearest-neighbour ligands (no charge transfer from the ligands to localized electronic levels of the atom with the core hole, as occurs with the main peak of the core-ionized heavier 3dn transition metal ions or the light lanthanide ions). To good approximation it is shown that: (1) the Auger parameter shift is a function of the number, distance, electronic polarizability and local symmetry of the first-neighbour ligands of the core-ionized atom; (2) the dipole-dipole interactions can play a key role in determining the extent of the extra-atomic relaxation energy and hence of the Auger parameter shift
Il primo articolo di Albert Einstein: tensione superficiale di liquidi organici e struttura molecolare
No full textThe first paper published by Albert Einstein in the Annalen der Physik in 1901 was related in same way to the molecular structure of organic substances. Einstein was interested ti characteriza the forces acting between the molecules and found support for the model he described in the manuscript from the experimental data on the surface tension of organic substances in the liquid state reported in contributions by Eotvos and by Ramsay and Shields. einstein found the data he needed following suggestions found in the third edition of the Grundriss der Allgemeinen Chemie (Compendium of General Chemistry) published by Ostwald in 1899. Einstein sent a reprint of his contribution to Ostwald in March 1901 togheter with a letter asking whether a physical-mathematical like himself could be useful for the researches that were going on his laboratory
Elettronegatività dei gas nobili secondo i modelli di calcolo di Pauling, Mulliken e Allen
No full textRiassunto – Il concetto di elettronegatività gioca un ruolo fondamentale in chimica. È di interesse conoscere l’elettronegatività dei gas nobili poiché, a causa della configurazione elettronica degli orbitali di valenza s2 (He) e s2 p6 (Ne, Ar, Kr, Xe, Rn), a prescindere dalla scala di elettronegatività utilizzata, l’intuito chimico suggerisce che essa dovrebbe essere molto inferiore rispetto all’elettronegatività di tutti gli altri atomi nella tavola periodica.
Contrariamente a questa aspettativa He e Ne sono gli atomi più elettronegativi della tavola periodica. Questi elementi sono anche i meno polarizzabili (i più «duri») e quindi potrebbero formare composti chimici solo sotto pressioni molto grandi, come suggerito da recenti lavori riportati in letteratura.Abstract - Electronegativity is one of the most important concepts in chemistry. It is of interest to know the electronegativity of the noble gases because at first, due to the filled s2 (He) and s2 p6 (Ne, Ar, Kr, Xe, Rn) valence electron shells, no matter the electronegativity scale employed, chemical intuition would suggest that it should be much lower with respect to the electronegativity of all the other atoms in the periodic table.
On the contrary, according to all the electronegativity scales, He and Ne are the most electronegative atoms in the periodic table. These elements are also the less polarizable (the most harder) and therefore chemical compounds could be synthesized only under very high pressure, as suggested by recent literature data
Il lavoro di Cannizzaro ad Alessandria (1851-1855): la chimica prima della legge degli atomi
No full textLe tre lettere di Cannizzaro pubblicate negli Annalen der Chemie und Pharmacie, nel biennio 1853-1854, tutte con lo stesso titolo, Ueber den der Benzoësäure entsprechenden Alkohol, annunciano alla comunità dei chimici la scoperta e le proprietà chimiche dell’alcole corrispondente all’acido benzoico (l’alcol benzilico). Le formule delle sostanze sono determinate applicando la legge degli equivalenti sia ai risultati dell’analisi elementare (C, H), sia alle misure di densità di vapore secondo il metodo di Dumas. Nell’arco di pochi anni da queste prime pubblicazioni Cannizzaro rivoluzionerà la chimica con la pubblicazione del Sunto di un corso di filosofia chimica, fatto nella R. Università di Genova (Nuovo Cimento Vol.7, pp.321-366, 1858), definendo chiaramente i concetti di molecola e di atomo e applicando magistralmente agli stessi risultati sperimentali l’ipotesi di Avogadro e la legge degli atomi.In the 1853-1854 years, Cannizzaro published three letters in the Annalen der Chemie und Pharmacie. These letters had all the same title, i.e. Ueber den der Benzoësäure entsprechenden Alkohol, and were written to describe the synthesis and the chemical properties of the benzil alcohol. The formulas of the substances were obtained according to the law of equivalents, starting from the elementary analyses of C and H and the density measurements of the vapors, as resulting from the Dumas’ method. Just a few years later these first publications, Cannizzaro published the work Sunto di un corso di filosofia chimica, fatto nella R. Università di Genova (Nuovo Cimento Vol.7, pp.321-366, 1858), through which he establish a kind of chemical revolution since for the first time a clear distinction between the concepts of molecule and atom is reported, and a very fine application of the Avogadro’s hypothesis and the law of atoms is made
The contribution of X-ray photoelectron and X-ray excited Auger spectroscopies in the characterization of zeolites and of metal clusters entrapped in zeolites
No full textX-ray photoelectron spectroscopy (X.p.s.) and X-ray-induced Auger spectroscopy (XAES) can be considered feasible techniques for the characterization of the chemical nature and surface composition of zeolites. In this review, we discuss the Auger parameter concept, an experimental quantity obtained combining X.p.s. and XAES measurements. We show that the Auger parameter is useful to overcome the problem of charging and to obtain structural and electronic information of metal clusters entrapped in zeolites. A simple electrostatic model is developed to calculate Auger parameter shifts as a function of number, distance, electronic polarizability, and local geometry of the ligand atoms around the core-ionized atom. Using our model, we investigated Pdm and Cum, clusters entrapped in Y and A zeolites, respectively. It is demonstrated that the Auger parameter shift of Pdm clusters depends not only on the nuclearity and geometry of the clusters, but also on the nature of the heteroatoms in its close vicinity. This result could be explained considering the influence of these heteroatoms on the electronic polarizability of the surface Pd atoms of the cluster
Auger parameter and Wagner plot in the characterization of chemical states: Initial and final state effects
No full textUsing the point-charge potential approximation, the Auger parameter and the Wagner plot, we present a simple semiquantitative model useful to rationalize the dependence of the local electronic structure on the atomic environment
The local and non-local screening state in photoelectron-spectroscopy studied by means of the Auger parameter and the chemical state plots
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