The Metallic State

There is a simple explanation of this change in properties in terms of the electronic structure of the metals. The potassium atom has only one electron outside of its completed argon shell. It could use this electron to form a single covalent bond with another potassium atom, as in the diatomic molecules K2 that are present, together with monatomic molecules K, in potassium vapor. In the crystal of metallic potassium each potassium atom has a number of neighboring atoms, at the same distance. It is held to these neighbors by its single covalent bond, which resonates 

A graph of the ideal density of the metals of the first long period. The ideal density is defined here as the density that these metals would have if their atomic masses were all equal to 50. 

It is interesting to note that in the metallic state chromium has metallic valence 6, corresponding to the oxidation number+6 characteristic of the chromates and dichromates, rather than to the lower oxidation number 

It is mentioned above that in potassium metal each atom, with one valence electron, can form one covalent bond, and that this bond is not between the atom and a single neighboring atom, but instead resonates among several positions. For four potassium atoms in a square, we might write two valence-bond structures  

These structures are analogous to the two Kekule structures for the benzene molecule. Resonance between the two structures would stabilize the metal relative to a crystal composed of K2 molecules, each with a fixed covalent bond. 

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The principal ha2ards of plutonium ate those posed by its radioactivity, nuclear critical potential, and chemical reactivity ia the metallic state. Pu is primarily an a-emitter. Thus, protection of a worker from its radiation is simple and usually no shielding is requited unless very large (kilogram) quantities are handled or unless other isotopes are present. 

Because no process has been developed for selectively removing impurities in vanadium and vanadium alloys in the metallic state, it is essential that all starting materials, in aggregate, be pure enough to meet final product purity requirements. In addition, the consoHdation method must be one that prevents contamination through reaction with air or with the mold or container material. 

Discharging to this lower cell voltage usually results ia shorter cycle life. Enough excess iron should be provided ia the cell design to avoid this problem. Active iron ia the metallic state is slowly attacked by the alkaline electrolyte according to... 

Cesium [7440-46-2] Cs, is a member of the Group 1 alkali metals. It resembles potassium and mbidium ia the metallic state, and the chemistry of cesium is more like that of these two elements than like that of the lighter alkaU metals. 

Cesium was first produced ia the metallic state by electrolysis of a molten mixture of cesium and barium cyanides (2). Subsequentiy the more common thermochemical—reduction techniques were developed (3,4). There were essentially no iadustrial uses for cesium until 1926, when it was used for a few years as a getter and as an effective agent ia reduciag the electron work function on coated tungsten filaments ia radio tubes. Development of photoelectric cells a few years later resulted ia a small but steady consumption of cesium and other appHcations for cesium ia photosensing elements followed. 

Copper [7440-50-8] Cu, critically important to the development of civilization, is the only common metal found naturally in the metallic state. It was thus suitable for the production of tools, and ancient people made use of its easy workabiUty and beauty. Furthermore, the ease with which the oxide can be reduced to the metal, together with the tendency of copper to alloy with other metals naturally present in the ores, promoted broad usage. 

The higher ionisation energy and smaller ionic radius of copper contribute to its forming oxides much less polar, less stable, and less basic than those of the alkah metals (13). Because of the relative instabiUty of its oxides, copper joins silver in occurring in nature in the metallic state. 

A guide to tire stabilities of inter-metallic compounds can be obtained from the semi-empirical model of Miedema et al. (loc. cit.), in which the heat of interaction between two elements is determined by a contribution arising from the difference in work functions, A0, of tire elements, which leads to an exothermic contribution, and tire difference in the electron concentration at tire periphery of the atoms, A w, which leads to an endothermic contribution. The latter term is referred to in metal physics as the concentration of electrons at the periphery of the Wigner-Seitz cell which contains the nucleus and elecUonic structure of each metal atom within the atomic volume in the metallic state. This term is also closely related to tire bulk modulus of each element. The work function difference is very similar to the electronegativity difference. The equation which is used in tire Miedema treatment to... 

Metallic materials consist of one or more metallic phases, depending on their composition, and very small amounts of nonmetallic inclusions. In the metallic state, atoms donate some of their outer electrons to the electron gas that permeates the entire volume of the metal and is responsible for good electrical conductivity (10 S cm ). Pure elements do not react electrochemically as a single component. A mesomeric state can be approximately assumed... 

An alternative approach to stabilizing the metallic state involves p-type doping. For example, partial oxidation of neutral dithiadiazolyl radicals with iodine or bromine will remove some electrons from the half-filled level. Consistently, doping of biradical systems with halogens can lead to remarkable increases in conductivity and several iodine charge transfer salts exhibiting metallic behaviour at room temperature have been reported. However, these doped materials become semiconductors or even insulators at low temperatures. 

Ruthenium and osmium are generally found in the metallic state along with the other platinum metals and the coinage metals. The major source of the platinum metals are the nickel-copper sulfide ores found in South Africa and Sudbury (Canada), and in the river sands of the Urals in Russia. They are rare elements, ruthenium particularly so, their estimated abundances in the earth s crustal rocks being but O.OOOl (Ru) and 0.005 (Os) ppm. However, as in Group 7, there is a marked contrast between the abundances of the two heavier elements and that of the first. 

In the concentrated ores most metals are in chemical compounds, as oxides or sulfides. Reducing these compounds to the metallic state in the final stage in producing metal can be accomplished by chemical processes or electrolysis. Two examples of chemical reduction are... 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

Table 21-IV. properties of the alkaline earths in the metallic state...

X-ray diffraction analysis of the spent catalyst (Table VI) revealed that the nickel was present only in the metallic state. Chemical analyses demonstrated very little difference in catalyst composition at the gas inlet and outlet. 

Here, for nearly all of the elements, the number of 4f electrons in the metallic state and in the trichloride is the same, so we expect a largely smooth energy... 

Suppose you prepared an iron oxide catalyst supported on an alumina support. Your aim is to use the catalyst in the metallic form, but you want to keep the iron particles as small as possible, with a degree of reduction of at least 50%. Hence, you need to know the particle size of the iron oxide in the unreduced catalyst, as well as the size of the iron particles and their degree of reduction in the metallic state. Refer to Chapters 4 and 5 to devise a strategy to obtain this information. (Unfortunately for you, it appears that electron microscopy and X-ray diffraction do not provide useful data on the unreduced catalyst.)... 

Besides these many cluster studies, it is currently not knovm at what approximate cluster size the metallic state is reached, or when the transition occurs to solid-statelike properties. As an example. Figure 4.17 shows the dependence of the ionization potential and electron affinity on the cluster size for the Group 11 metals. We see a typical odd-even oscillation for the open/closed shell cases. Note that the work-function for Au is still 2 eV below the ionization potential of AU24. Another interesting fact is that the Au ionization potentials are about 2 eV higher than the corresponding CUn and Ag values up to the bulk, which has been shown to be a relativistic effect [334]. A similar situation is found for the Group 11 cluster electron affinities [334]. 

Controlled decomposition of pre-formed [(COD)Pt(CH3)2] in the presence of triorganoaluminium led to the preparation of the first Pt cluster (size 0.75 + 0.1 nm). The one-shell structure and the metallic state were confirmed by XPS and XANES [352]. 

The metal size clearly increases when the decomposition takes place on the substrate. Nevertheless, the overall shift after complete decomposition is the same both on crystalline and amorphous substrates. This can be explained by the assumption that the increase of the number of the metal atoms in the cluster takes place also on an amorphous substrate, on a scale high enough to shift the core levels but low enough to maintain a constant emitted intensity ratio between the substrate and the metal core levels. The authors concluded therefore that the core-level position is highly size-sensitive in the range of very small particles, e.g. < 100 atoms where the associated electronic properties are primarily atomic. However, on approaching the metallic state for >100 atoms, the corelevel shift is a much poorer criterion of the cluster size. 

It is most convenient to explain catalysis using an example. We have chosen a hydrogenation catalysed by nickel in the metallic state. According to the schematic of Fig. 3.1 the first step in the actual catalysis is adsorption . It is useful to distinguish physisorption and chemisorption . In the former case weak, physical forces and in the latter case relatively strong, chemical forces play a role. When the molecules adsorb at an active site physisorption or chemisorption can occur. In catalysis often physisorption followed by chemisorption is the start of the catalytic cycle. This can be understood from Fig. 3.2, which illustrates the adsorption of hydrogen on a nickel surface. 

Figure 3.8 shows a variety of materials. This, of course, is not surprising. In principle, the composition of a solid material will depend on several factors, including the reaction conditions. For example, when an oxidation is carried out a metal like Co will be in the metallic state or the oxidized. state depending on the reaction conditions. When used in chlorination many systems will be present as chlorides. 

Fe(6-Mepy)2(py)tren] (004)2 Doped in PSS. Magnetic susceptibilities measured for a microcrystalline sample of the complex produce a magnetic moment value = 0.36 pg at 10 K and 0.61 pg at 150 K, followed by a gradual increase to Peff = 2.80 pe at 311 K [138]. Thus 26% of the complexes are in the HS state at 300 K if a magnetic moment of 5.1 Pe is assumed for the pure HS compound. On the other hand, the complex doped into a polystyrene sulfonate (PSS) film does not provide any evidence for a thermal population of the HS state up to 340 K as demonstrated by variable-temperature UV-VIS and Mossbauer spectra. In fact, all the complexes doped into the PSS film are in the LS state at temperatures below 340 K. However, if irradiated by a single pulse of a Q-switched Nd/YAG laser (532 mp), the complex is excited from the LS ground state to the HS J2 states via an intermediate MLCT state and the metal states. The subsequent back relaxation from the excited T2 state to the... 

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