Early Experiments to Characterize the Atom

A cathode-ray tube. The fast-moving electrons excite the gas in the tube, causing a glow between the electrodes. The green color in the photo is due to the response of the screen (coated with zinc sulfide) to the electron beam. 

On the basis of the work of Dalton, Gay-Lussac, Avogadro, Cannizzaro, and others, chemistry was beginning to make sense. The concept of atoms was clearly a good idea. Inevitably, scientists began to wonder about the nature of the atom. What is an atom made of, and how do the atoms of the various elements differ  

Deflection of cathode rays by an applied electric field. 

Ernest Rutherford (1871-1937) was born on a farm in New Zealand. In 1895 he placed second in a scholarship competition to attend Cambridge University but was awarded the scholarship when the winner decided to stay home and get married. As a scientist in England, Rutherford did much of the early work on characterizing radioactivity. He named the a and /3 particles and the g ray and coined the term half-life to describe an important attribute of radioactive elements. His experiments on the behavior of a particles striking thin metal foils led him to postulate the nuclear atom. He also invented the name proton for the nucleus of the hydrogen atom. He received the Nobel Prize in chemistry in 1908. 

Rutherford s experiment on a-partide bombardment of metal foil. (Gold foil was used in the original experiments because it can be hammered into extremely thin sheets.) 

The first important experiments that led to an understanding of the composition of the atom were done by the English physicist J. J. Thomson (Fig. 2.6), who studied electrical discharges in partially evacuated tubes called cathode-ray tubes (Fig. 2.7) during 

Unless otherwise noted, all art on this page is ) Cengage Learning 2014. 

A classic English plum pudding in which the raisins represent the distribution of electrons In the atom. 

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The part of PrP that is structurally best characterized is the C-terminal domain (residues 121—231), often referred to as the structured or folded domain. By contrast, the N-terminal part of the molecule is highly flexible, and early nuclear magnetic resonance (NMR) experiments suggested that it is largely disordered [5, 6]. However, recent studies have made substantial advances into the structural understanding of parts of this domain. Below we describe the current state of knowledge about the atomic-level structures of the N- and C-terminal domains. 

ReflEXAES can be used for near-surface structural analysis of a wide variety of samples for which no other technique is appropriate. As with EXAES, ReflEXAES is particularly suited for studying the local atomic structure around particular atomic species in non-crystalline environments. It is, however, also widely used for the analysis of nanocrystalline materials and for studying the initial stages of crystallization at surfaces or interfaces. ReflEXAES was first proposed by Barchewitz [4.135], and after several papers in the early nineteen-eighties [4.136, 4.168-4.170] it became an established (although rather exotic) characterization technique. Most synchrotron radiation sources now have beam-lines dedicated to ReflEXAES experiments. 

In the early 1990s Raman spectroscopy was applied to the characterization of TS-1 catalysts [55,56]. In such experiments, beside the 960 cm band, already observed by IR spectroscopy (see Sect. 3.5), a new component at 1125 cm was detected by Scarano et al. [55] (see Fig. 2f). The 1125 cm band was recognized to be a fingerprint of the insertion of Ti atoms in the ze-olitic framework [55]. This band could not be observed in the IR studies as totally overshadowed by an extremely intense band around 1000 cm due to Si02 framework modes (Fig. 2e). 

Some of the forms of isomerism have little more than historic interest now, as their significance has diminished with the rise in physical methods that makes their identification and origins routine, and no longer involves the demanding experiments of an earlier era to probe their form. Nevertheless, some remain important, and others at least give a flavour of the historical development of the field, and this deserves a brief discussion. Constitutional isomers are characterized by species of the same empirical formula (which was able to be determined at an early date in the development of the field) but clearly different physical properties associated with different atom connectivity. 

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