Structures of the elements

The structure of p-rhombohedral B consists of Bg4-units, coimected through Bjo-units. Each Bg4-unit is conveniently viewed in terms of the sub-units shown in Fig. 13.7. Their interrelationship is described in the figure caption, but an interesting point to note is the structural relationship between the Bgo-sub-unit shown in Fig. 13.7c and the fuUerene C o (Fig. 14.5). The covalent lattices of both a- and p-rhombohe-dral B are extremely rigid, making crystalline B very hard, with a high melting point (2453 K for p-rhombohedral B). 

Boron is inert under normal conditions except for attack by F2. At high temperatures, it reacts with most non-metals (exceptions include H2), most metals and with NH3. The formations of metal borides (see Section 13.10) and boron nitride (see Section 13.8) are of particular importance. 

The reactivities of the heavier group 13 elements contrast with that of the first member of the group. Aluminium readily oxidizes in air (see above). It dissolves in dilute mineral acids (e.g. reaction 13.3) but is passivated by concentrated HNO3. Aluminium reacts with aqueous NaOH or KOH, liberating H2 (eq. 13.4). 

Reactions of A1 with halogens at room temperature or with N2 on heating give the Al(lll) halides or nitride. Aluminium is often used to reduce metal oxides, e.g. in the thermite process (eq. 13.5) which is highly exothermic. 

Gallium, indium and thallium dissolve in most acids to give salts of Ga(III), In(III) or T1(I), but only Ga liberates H2 from aqueous alkali. All three metals react with halogens at, or just above, 298 K. The products are of the type MX3 with the exceptions of reactions 13.6 and 13.7. 

5 Part of one layer of the infinite lattice of a-rhombohedral boron, showing the Bn-icosahedral building blocks which are covalently linked to give a rigid, infinite lattice. 

VM Fig. 13.6 The construction of the Bg -unit, the main building block of the infinite lattice of P-rhombohedral boron, (a) In the centre of the unit is a B] 2-icosahedron, and (b) to each of these twelve, another boron atom is covalently bonded, (e) A B o-cage is the outer skin of the Bg4-unit. (d) The final Bg -unit can be described in terms of covalently bonded sub-units (Bi2)(Bi2)(Bgo). 

Boron is inert under normal conditions except for attack by F2. At high temperatures, it reacts with most non-metals 

Hydrogen, nitrogen, oxygen,end the halogens bind as dimers that are then weaxly held together on the lattices which are indicated by the Pearson symbol. 

Element Common name Pearson symbol fersen symbol 

The most frequently occurring open structures types are shown in Fig. 1.2. 

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The wave function T i oo ( = 11 / = 0, w = 0) corresponds to a spherical electronic distribution around the nucleus and is an example of an s orbital. Solutions of other wave functions may be described in terms of p and d orbitals, atomic radii Half the closest distance of approach of atoms in the structure of the elements. This is easily defined for regular structures, e.g. close-packed metals, but is less easy to define in elements with irregular structures, e.g. As. The values may differ between allo-tropes (e.g. C-C 1 -54 A in diamond and 1 -42 A in planes of graphite). Atomic radii are very different from ionic and covalent radii. 

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

The layer and chain structures of the elements of the fifth and sixth main groups result by contraction of certain distances in the a-Po structure (stereo images)... 

Table 14.1 Numbers of structures of the elements known until 2006 in the solid state at different conditions...

J. Donohue, The Structures of the Elements. John Wiley Sons, Inc., 1974. 

The allotropy of carbon is due to variations in the crystal structure of the element. There are three allotropes of carbon graphite, diamond, and... 

While some chemists were busy breaking down various substances available to them in an attempt to discover new elements, others were probing into the structures of the elements themselves. In 1804, John Dalton, an English chemist, proposed a theory to explain some of the known properties of elements. Dalton s theory stated that each element, indeed, all matter, consisted of huge... 

The ordering principle of this section is different to the preceding section on the isolated molecules or molecular ions. We start with the structures of the elements and then classify the polymeric Te structures by the number of atoms in the constituting homocycles, i.e. four-, five- and six-membered rings. 

Despite this, proven rules for boron clusters can be applied to the smaller metalloid Al, Ga, and In clusters with certain additional assumptions, as recent DFT calculations have shown [87]. In addition, counting rules for smaller Ga and Al metalloid clusters have been developed [123], which will, however, probably not be transferable to the larger clusters. Therefore the first assignment principle presented here for the larger metalloid clusters incorporates the structures of the elements in the various modifications, which means that the metalloid or elementoid clusters are described as nanostructured element modifications. 

Notice that in the following only a selection is presented of the crystal structures of the elements of the 14th to 17th groups. A few more details and data are reported in the specific paragraphs of Chapter 5. 

By the time Bohr turned his attention to the problem, significant advances had been made. Physicists working with the old quantum theory had developed a number of rules about the manner in which electrons interacted with one another. Bohr realized that these rules could be used to confirm Kossel s hypothesis and to make informed guesses about the atomic structure of the elements. For example, hydrogen has one electron, placed in the innermost shell. Helium, having two electrons, has this shell filled up. Thus lithium, the third element, has to have two electrons in an inner shell and one with an... 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

In the previous example, zinc was more easily oxidized than copper. The ability of one element to donate electrons to another element is based on the electron structure of the element and energy considerations. A spontaneous redox reaction, like any spontaneous reaction, results in a more stable configuration of the chemical system. The following list of elements shows how easily an element is oxidized. 

Table 1.1 gives the structures of the elements at zero temperature and pressure. Each structure type is characterized by its common name (when assigned), its Pearson symbol (relating to the Bravais lattice and number of atoms in the cell), and its Jensen symbol (specifying the local coordination polyhedron about each non-equiyalent site). We will discuss the Pearson and Jensen symbols later in the following two sections. We should note,... 

The concepts required for understanding the bonding and structure of the elements and binary compounds are most easily introduced by considering first the nature of the chemical bond in small molecules. We will use theory... 

In the latter, the valency angles must be about 100°, so the layers cannot be flat. Their shape is obtained if, in Figure 38, the atoms shown with the clear circles are displaced somewhat below the plane of the paper and the shaded ones similarly, above it. If the layers formed in this way are then arranged on top of one another, the crystal structure of the elements arsenic, antimony and bismuth are obtained in their normal forms in which they have metallic properties. There also exists a modification of phosphorus with a similar structure. In addition, there are other forms of arsenic and antimony, the properties of which correspond to those of yellow phosphorus these forms contain molecules p As4 and Sb4. 

Tellurium melts at 452° C.1 and boils near 1390° C. under ordinary pressure,2 but volatilises at as low a temperature as 430° C. in a cathode-ray vacuum the vapour is yellow in colour.3 Like the density, the specific heat of the solid is inconstant, ranging from 0-0475 for the distilled element to 0-0524 for the precipitated amorphous substance.4 It has been observed 5 that exposure to X-rays increases the specific heat of tellurium by about 8 per cent., possibly owing to a change in the structure of the element. 

The classical theory of Hume-Rothery states that a difference in atomic diameters of solute and solvent atoms of more than 15% produces restricted solid solubility. The closest distance of approach of the atoms in the crystals of the element is taken as a measure of the atomic size. Substitution of a larger atom into a lattice requires a high amount of energy due to the concomitant disorganization of the parent lattice. However, the size factor becomes less important [221] when the difference in size is 8% or less. It is desirable (though not essential) that the size factor and the crystal structure of the elements producing a solid solution in all proportions be favourable. It is, however, apparent that if elements forming alloys did not possess the same crystal structure, a continuous series of solid solutions would be impossible. 

In retrospect, with a much fuller understanding of the underlying electronic and particle structure of the elements, most aspects of the periodicity of the elements come as no surprise, but the fact remains that... 

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