Element compound, formation

The important (3-stabilizing alloying elements are the bcc elements vanadium, molybdenum, tantalum, and niobium of the P-isomorphous type and manganese, iron, chromium, cobalt, nickel, copper, and siUcon of the P-eutectoid type. The P eutectoid elements, arranged in order of increasing tendency to form compounds, are shown in Table 7. The elements copper, siUcon, nickel, and cobalt are termed active eutectoid formers because of a rapid decomposition of P to a and a compound. The other elements in Table 7 are sluggish in their eutectoid reactions and thus it is possible to avoid compound formation by careful control of heat treatment and composition. The relative P-stabilizing effects of these elements can be expressed in the form of a molybdenum equivalency. Mo (29) ... 

Barium sulfide solutions undergo slow oxidation in air, forming elemental sulfur and a family of oxidized sulfur species including the sulfite, thiosulfate, polythionates, and sulfate. The elemental sulfur is retained in the dissolved bquor in the form of polysulfide ions, which are responsible for the yellow color of most BaS solutions. Some of the mote highly oxidized sulfur species also enter the solution. Sulfur compound formation should be minimized to prevent the compounds made from BaS, such as barium carbonate, from becoming contaminated with sulfur. 

Mendeleev s genius lay in recognizing that just as It was the element in the abstract sense that survived intact in the course of compound formation, so atomic weight was the only quantity that survived in measurable amounts. He therefore took the step of associating these two features an element was to be characterized by its atomic weight. In a sense an abstract element had acquired a single measurable attribute that would remain unchanged... 

A compound has a fixed composition, whereas the composition of a mixture may be varied. There are always two H atoms for each O atom in a sample of the compound water, but sugar and sand, for instance, can be mixed in any proportions. Because the components of a mixture are merely mingled with one another, they retain their own chemical properties in the mixture. In contrast, a compound has chemical properties that differ from those of its component elements. The formation of a mixture is a physical change, whereas the formation of a compound requires a chemical change. The differences between mixtures and compounds are summarized in Table G.l. 

Figure 1. Formation of ternary borides MreMj3B2 and different structure types (Mre = rare-earth element, M-p = transition-metal element). , CeCo3B2 type ErIr3B2 type O, URujBj type El, Ndo7,Rh3 29B2 type IS, YOS3B2 type B, Laofi3Rh3B2 type , compound formation observed, but structure type unknown. Refs a , b , c , d e , f g , h , i , j ", k , 1 , m , r, s - , t u = 45 see also ref. 62.

Formation of Alloys Between Group-IA Elements 7.2.4.1. Binary Alloys 7.2.4.I.2. Compound Formation. 

Traditional solid-state synthesis involves the direct reaction of stoichiometric quantities of pure elements and precursors in the solid state, at relatively high temperatures (ca. 1,000 °C). Briefly, reactants are measured out in a specific ratio, ground together, pressed into a pellet, and heated in order to facilitate interdiffusion and compound formation. The products are often in powdery and multiphase form, and prolonged annealing is necessary in order to manufacture larger crystals and pure end-products. In this manner, thermodynamically stable products under the reaction conditions are obtained, while rational design of desired products is limited, as little, if any, control is possible over the formation of metastable intermediates. ... 

On the other hand polysilylalkynes with phenyl or allyl substituents are converted with triflic acid into polymeric alkynylsilyltriflates. These polymers react with many acidic element hydrogen compounds or lithium element compounds with formation of silicon element bonds. Thus we found an easy approach to numerous new functional substituted alkynes [12], Eq.(9) shows selected examples of this reaction type. 

Oxidative UPD involves the oxidation of species to form an atomic layer where the precursor contains the element in a negative oxidation state. A classic example is the formation of oxide layers on Pt and Au, where water is oxidized to form atomic layers of oxygen. Halide adsorption can be thought of similarly, where a species such as I oxidatively adsorbs on a metal surface as the halide atom. In that case, a bulk film is not formed at more positive potentials, but the diatomic is generated and diffuses into solution. With respect to compound formation, oxidative UPD from a sulfide solution is a good example ... 

Data for Mo and W compounds (MoM and WM ) are included in Figure 7.13 to show the effect of going from one period to the next. The variation in enthalpy of formation with the difference in number of valence electrons is similar however, the enthalpies of formation are more exothermic for the MoM and WM compounds compared with the corresponding first transition metal series element compounds CrM. Finally, it should be added that the enthalpies of formation of equiatomic alloys of elements of the same group are close to zero. 

Second rule Effect of the electrochemical nature. If the electrochemical characteristics of the two elements are similar, solid solution formation may be expected, otherwise compound formation is more probable. 

In Figs. 2.22 and 2.23 all the binary combinations are mapped as a function of the Mendeleev numbers of the two elements involved. The compound formation capability is represented in Fig. 2.22 by means of a few codes, whereas in Fig. 2.23 an indication is given of the thermal stability of the intermediate phases. To this end, values correlated to the so-called Raynor Index (Raynor 1972, 1974) are coded in this figure. 

Figure 2.22. Compound formation capability in binary systems. The different element combinations are mapped on Mendeleev number coordinates and those systems are indicated in which the formation of intermediate phases has been observed (either from the liquid or in the solid state). Blank boxes indicate systems for which no certain data are available. Notice that the compound-forming alloys are crowded in a region corresponding to a large difference in the Mendeleev numbers of the elements involved (for instance, basic metals with semi-metals).

Phase diagrams of alkali metal alloys. The pattern of the intermetallic reactivity of these metals is shown in Fig. 5.6, where the compound formation capability with the different elements is summarized. 

Figure 5.6. Compound formation capability in the binary alloys of alkali metals. The different elements, the binary combinations of which with Li, Na, K, Rb, Cs are considered, are identified by their positions in the Periodic Table. No rehable data have been found about the stable equilibrium phases in the Na-P and Cs-As systems compound formation is, however, probable.

Figure 5.7. Binary compound formation capability of Ca, Sr, Ba and of Eu and Yb. Those elements are marked for which compounds with the mentioned divalent metals are known.

Plutonium. The compound formation pattern of this metal shows several analogies with that of uranium. Compound formation is systematically observed with the elements from the 7th group on and, with a few intermediate phases, in the systems with some elements of the very first groups. 

An indication of the trend of the solid phase stability in the alloys of Mn and Re with the different elements of the 4th and 6th rows of the Periodic Table is contained in Table 5.39, where the melting points of selected compounds have been collected. In the Mn series alloys we may notice, here too, the gaps in the pattern of the compound formation. In the case of Re alloys, very high melting points are observed in the compounds with other refractory metals (even if often... 

Intermetallic chemistry of Be, Mg, Zn, Cd and Hg 5.12.4.1 Phase diagrams of the Be, Mg, Zn, Cd and Hg alloys. The systematics of the compound formation of these metals in their binary alloys with the different elements is summarized in Fig. 5.33. On the overall they give a rather complex picture even so a number of relationships and similarities between various pairs of metals may be singled out. To go into this point in more detail, in the same figure a comparison has also been made with the compound formation patterns of Ca and A1 which are described in 5.4 and 5.13 but are close in the Periodic Table to the metals here considered. The similarity between the Be and Zn patterns may be underlined, as also that between Be and Al, being an example of the so-called diagonal relationships presented in 4.2.2.2. 

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