Elements allotropic modifications

Arsenic and antimony resemble phosphorus in having several allotropic modifications. Both have an unstable yellow allotrope. These allotropes can be obtained by rapid condensation of the vapours which presumably, like phosphorus vapour, contain AS4 and Sb4 molecules respectively. No such yellow allotrope is known for bismuth. The ordinary form of arsenic, stable at room temperature, is a grey metallic-looking brittle solid which has some power to conduct. Under ordinary conditions antimony and bismuth are silvery white and reddish white metallic elements respectively. 

AUotropes. Systematic names for gaseous and liquid modifications of elements are sometimes needed. Allotropic modifications of an element bear the name of the atom together with the descriptor to specify the modification. The following are a few common examples ... 

Boron is unique among the elements in the structural complexity of its allotropic modifications this reflects the variety of ways in which boron seeks to solve the problem of having fewer electrons than atomic orbitals available for bonding. Elements in this situation usually adopt metallic bonding, but the small size and high ionization energies of B (p. 222) result in covalent rather than metallic bonding. The structural unit which dominates the various allotropes of B is the B 2 icosahedron (Fig. 6.1), and this also occurs in several metal boride structures and in certain boron hydride derivatives. Because of the fivefold rotation symmetry at the individual B atoms, the B)2 icosahedra pack rather inefficiently and there... 

The substance indicated by the same symbol in two or more equations is in exactly the same state in the reactions represented by those equations. In particular, the different allotropic modifications of a solid element (e.g., charcoal, graphite, diamond or yellow and red phosphorus) have different heats of combustion, and the particular form used must be specified in every case. 

The only element that was discovered in body fluids (urine). This is plausible, as P plays a main role in all life processes. It is one of the five elements that make up DNA (besides C, H, N, and 0 evolution did not require anything else to code all life). The P-O-P bond, phosphoric acid anhydride, is the universal energy currency in cells. The skeletons of mammals consists of Ca phosphate (hydroxylapatite). The element is encountered in several allotropic modifications white phosphorus (soft, pyrophoric P4, very toxic), red phosphorus (nontoxic, used to make the striking surface of matchboxes), black phosphorus (formed under high pressures). Phosphates are indispensable as fertilizer, but less desirable in washing agents as the waste water is too concentrated with this substance (eutrophication). It has a rich chemistry, is the basis for powerful insecticides, but also for warfare agents. A versatile element. 

According to the atomic theory the differences between what are termed "allotropic modifications" are generally ascribed to differences in the number and arrangement of the atoms constituting the molecules of such "modifications," and not to any differences in the atoms themselves. But we cannot argue that two such "allotropic modifications" or elements which are transmutable into one another... 

Amorphous Selenium.—(1) Vitreous Selenium.—When molten selenium is cooled in not too protracted a manner, no definite solidification or crystallisation ensues, but the mass gradually hardens and the product really represents a strongly undercooled liquid like glass. Vitreous selenium is a brittle reddish-brown substance, exhibiting a conchoidal fracture. When finely powdered and viewed in thin layers it has a deep red colour. This form has an average density of 4-28 5 the value varies slightly, possibly owing to the presence of other allotropic modifications of the element. 

Plutonium was the first element to be synthesized in weighable amounts (6,7). Technetium, discovered in 1937, was not isolated until 1946 and not named until 1947 (8). Since the discovery of plutonium in 1940, production has increased from submicrogram to metric ton quantities. Because of its great importance, more is known about plutonium and its chemistry than is known about many of the more common elements. The metallurgy and chemistry are complex. Metallic plutonium exhibits seven allotropic modifications. Five different oxidation states are known to exist in compounds and in solution. 

The reader is invited to consider other possible allotropic modifications of the Group 14 elemental substances, and to become satisfied that none is likely to be more stable than the structures observed. 

Allotropic forms of carbon. In the solid state, the element carbon exists in three different allotropic modifications—amorphous carbon and the two crystalline forms known as diamond and graphite. Amorphous carbon includes numerous common products such as wood charcoal, bone black, coke, lamp black, and carbon black. Each of these varieties of crystalline and amorphous carbon possesses properties that render it useful for a variety of purposes. 

From the perspective of structural chemistry, the modes of bonding, coordination, and the bond parameters of a particular element in its allotropic modifications may be further extended to its compounds. Thus organic compounds can be conveniently divided into three families that originate from their prototypes aliphatic compounds from diamond, aromatic compounds from graphite, and fullerenic compounds from fullerenes. 

To illustrate the type of analysis that is involved we exhibit a representative set of heat capacity data in Fig. 1.20.2 for oxygen, as a plot of CP versus log T this representation is useful for the direct calculation of the entropy of oxygen from the area under the curves. Note that the element exists in three allotropic modifications in the solid state, with transition temperatures near 23.6, 43.8, and 54.4 K, the last being the melting point of solid phase I. The boiling point of liquid oxygen is near 90.1 K. An extrapolation procedure was used below 14 K. 

By way of illustrations we display in Fig. 1.17.2a plot of the molar heat capacity of oxygen under standard conditions. The plot of Cp vs. In T is then used to determine the entropy of oxygen from the area under the curves. Note that the element in the solid state exists in three distinct allotropic modifications, with transition temperatures close to 23.6 and 43.8 K the melting point occurs at 54.4 K, and the boiling point is at 90.1 K. All the enthalpies of transition at the various phase transformations are accurately known. An extrapolation procedure was employed below 14 K, which in 1929 was about the lower limit that could conveniently be reached in calorimetric measurements. 

There is another fact which shows us that the law of Dulong and Petit cannot be strictly true. Many elements are capable of existing in several modifications which are distinguished from one another by their crystalline form, by their specific gravity, and by many other physical properties. These allotropic modifications have not the same specific heat, so that the atomic heat is not even constant for one and the same element. Still less, then, can we expect it to be constant for all elements. Wigand has shown that the denser modification of an element has always the smaller specific heat, as may be seen in the next table. 

A homogeneous chemical substance, for example an element such as sulphur or a compound such as water, may exist in several, and must be capable of existing in at least three different forms, viz. as gas, as liquid, and as solid. In many cases the solid may exist in various allotropic modifications, which differ from one another in crystalline form, melting point, density, specific heat, and, in fact, in all their ph5rsical properties. Every portion of matter which is in itself homogeneous, i.e. in which the smallest visible particles are exactly alike, and which is therefore separated in space from every other homogeneous but dissimilar portion of matter, was called by Willard Gibbs a phase. ... 

IR-3.4.1 Name of an element of indefinite molecular formula or structure IR-3.4.2 Allotropes (allotropic modifications) of elements IR-3.4.3 Names of allotropes of definite molecular formula IR-3.4.4 Crystalline allotropic modifications of an element... 

Allotropic modifications of an element bear the name of the atom from which they are derived, together with a descriptor to specify the modification. Common descriptors are Greek letters (a, (3, y, etc.), colours and, where appropriate, mineral names (e.g. graphite and diamond for the well known forms of carbon). Such names should be regarded as provisional, to be used only until structures have been established, after which a rational system based on molecular formula (see Section IR-3.4.3) or crystal structure (see Section IR-3.4.4) is recommended. Common names will continue to be used for amorphous modifications of an element and for those which are mixtures of closely related structures in their commonly occurring forms (such as graphite) or have an ill-defined disordered structure (such as red phosphoms) (see Section IR-3.4.5). 

Crystalline allotropic modifications are polymorphs of the elements. Each can be named by adding the Pearson symbol (see Section IR-11.5.2)7 in parentheses after the name of the... 

Allotropic modifications (allotropes) Different forms of the same element in the same physical state. 

The particular advantage of diffraction analysis is that it discloses the presence of a substance as that substance actually exists in the sample, and not in terms of its constituent chemical elements. For example, if a sample contains the compound A By, the diffraction method will disclose the presence of A B as such, whereas ordinary chemical analysis would show only the presence of elements A and B. Furthermore, if the sample contained both A B, and Aj Bjy, both of these compounds would be disclosed by the diffraction method, but chemical analysis would again indicate only the presence of A and B. To consider another example, chemical analysis of a plain carbon steel reveals only the amounts of iron, carbon, manganese, etc., which the steel contains, but gives no information regarding the phases present. Is the steel in question wholly martensitic, does it contain both martensite and austenite, or is it composed only of ferrite and cementite Questions such as these can be answered by the diffraction method. Another rather obvious application of diffraction analysis is in distinguishing between different allotropic modifications of the same substance solid silica, for example, exists in one amorphous and six crystalline modifications, and the diffraction patterns of these seven forms are all different. 

Some elements exist in more than one form. Familiar examples include (1) oxygen, found as O2 molecules, and ozone, found as O3 molecules, and (2) two crystalline forms of carbon—diamond and graphite (Figure 13-33). Different forms of the same element in the same physical state are called allotropic modifications, or allotropes. 

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