Elemental black phosphorus

Fhst, the number of valence functions (or frontier orbitals) and valence elechons (frontier orbital occupancy) determines the tendency toward cluster bonding. It is instructive to recall that the structural motif in elemental boron is the icosahedron with six-connected boron atoms see Borides Solid-state Chemistry), it is the tetrahedral carbon atom in the diamond form of elemental carbon with four-coimected carbon atoms and it is three-connected phosphorus atoms in the sheets of elemental black phosphorus (see Phosphides Solid-state Chemistry). Boron has more valence orbitals than valence elechons, naturally leading to orbitally rich cluster formation for example, BH has three orbitals and two elechons and forms... 

Just as the half-filled one-dimensional chains of the previous section underwent a Peierls distortion, so we expect that the simple cubic lattice with three p electrons would be similarly unstable. A distortion of this type has indeed occurred when we look at the observed structures of elemental black phosphorus (13.46) and arsenic (13.47). Alternate linkages have been broken along all three directions leaving each atom pyramidaUy three coordinate. The result is in accord with their atomic conjuration. At the simple cubic structure Px,Py and p bands would each be... 

The most thermod3Tiamically stable form of the element is black phosphorus, which can be prepared by heating red phosphorus under high pressure. The black form contains chains of P4 units cross-linked by P—P bonds, making this form even more polymerized and less strained than red phosphoms. Example explores another difference between the elemental forms of phosphorus. 

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. 

Although black phosphorus is generally inert, red phosphorus and particularly white phosphorus are capable of undergoing reactions to generate organophosphorus compounds. Several different sets of conditions may be used to form C-P bonds from elemental phosphorus. These will be discussed in the following sections. 

Although phosphorus is in group 15 with some other metalloids, it is usually classed as a nonmetal since it resembles nitrogen somewhat, the element above it in group 15. Both are essential to the biochemical field as vital elements to support life. Phosphorus has 10 known allotropic forms. This is an unusually high number for any element. A system of categorizing the allotropes by three colors has made it easier to keep track of them. These three colors are white, red, and black phosphorus. 

The stable form of nitrogen at room temperature is N2, which has an extraordinarily strong (946 kJ mol-1) triple bond In contrast, white phosphorus consists of P4 molecules (see Chapter 16), and the thermodynamically stable form is black phosphorus, a polymer. At temperatures above 800 °C dissociation to P> molecules does take place, but these are considerably less stable than N2 with a bond energy of488 kJ mol 1. In this case. too. in the heavier element several single bonds arc more effective than the multiple bond. 

The polymers which have been used to illustrate problems of inorganic polymer formation have been lieteroatomic. that is, their chains are built from different atoms alternating with each other. The other structure mentioned has been homoatomic—all the atoms in the chain are the same. There aie only a few homoatomic polymers of airy promise. Most elements will form only cyclic materials of low molecular weight if they polymerize at all. In addition to the silane polymers, black phosphorus, a high-pressure modification of the element, forms in polymeric sheets. 

There are three allotropic forms of elemental phosphorus white, red, and black phosphorus. At room temperature, pure white phosphorus is a tetrahedral crystal with a molecular formula of P4. In the pure form, white phosphorus is an ivory-colored, waxy solid. The commercial product is 99.9% pure and may have a slightly yellow color. In the literature, the commercial product is often referred to as yellow phosphorus. In this chapter, the terms white phosphorus and phosphorus are used to refer to P4, which includes white and yellow phosphorus. 

Elemental phosphorus exists in several allotropic forms (Van Wazer 1982). The best known and most important commercially is the a-white phosphorus whose properties are given in Table 3-2. Commercial white phosphorus is 99.9% pure, with a slight yellow color caused by traces of red phosphorus impurities. Hence, white phosphorus also is known as yellow phosphorus. When a-white phosphorus is cooled below -79.6°C, P-white phosphorus forms. Other important solid allotropes of phosphorus are red and black phosphorus (Van Wazer 1982). 

Black phosphorus is thermodynamically the most stable form of the element and exists in three known crystalline modifications orthorhombic, rhombohedral, and cubic, as well as in an amorphous form. Unlike white phosphorus, the black forms are all highly polymeric, insoluble, and practically non-flammable, and have comparatively low vapor pressures. The black phosphorus varieties represent the densest and chemically the least reactive of all known forms of the element. 

There are several allotropic forms of elemental phosphorus, the most common being the white, red, and black forms. Red phosphorus, which itself includes several forms, is obtained by heating the white form at 400 °C for several hours. An amorphous red form may also be prepared by subjecting white phosphorus to ultraviolet radiation. In the thermal transformation, several substances function as catalysts (e.g., iodine, sodium, and sulfur). Black phosphorus appears to consist of four different forms. These are obtained by the application of heat and pressure to the white form. The major uses of elemental phosphorus involve the production of phosphoric acid and other chemicals. Red phosphorus is used in making matches, and white phosphorus has had extensive use in making incendiary devices. Several of the important classes of phosphorus compounds will be discussed in later sections. 

Electrical Conductivity.—Phosphorus is an electrical insulator. The conductivity of the solid element was found to be of the order of 10-11 mhos per centimetre cube and that of the liquid 10-6 mhos per centimetre cube.11 Black phosphorus, which must be considered the... 

An interesting three-dimensional instance of a Peierls distortion at work (from one point of view) is the derivation of the observed structures of elemental arsenic and black phosphorus from a cubic lattice. This treatment is due to Burdett and coworkers.6 74 The two structures are shown in their usual representation in 96. It turns out that they can be easily related to a simple cubic structure, 97. 

Figure 4. The atomic density, or molarity, of the principal allotropes of the elements as a function of atomic number Z at room temperature and atmospheric pressure. The values for the noble gases have been extrapolated to STP. For sulfur, the values are shown with increasing molarity in the order S,, c-S8, c-S7, and and c-S6. The values for phosphorous correspond to white and black phosphorus, respectively, in the order of increasing molarity. Lines corresponding to covalence 2 and 3 have been drawn for the rare-earth metals and for the actinide elements. From [50].

These elements have fewer allotropic forms than phosphorus. For As and Sb, unstable yellow allotropes comparable to white phosphorus are obtainable by rapid condensation of vapors. They readily transform to the bright, metallic a-rhombo-hedral forms similar to rhombohedral black phosphorus. This is also the commonest form for bismuth. Other reported allotropes are not well characterized. 

Black phosphorus is actually thermodynamically the most stable form of the element and may be prepared in both... 

Phosphoms exists in at least three allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The three main allotropes are named for their colors white phosphorus (also called yellow phosphorus), red phosphorus, and black phosphorus (also called violet phosphorus). These allotropes have different physical and chemical properties. 

Black phosphorus is a third form of the element it has a network structure in which each P atom is bonded to three others at approximately right angles. 

CAS 7723-14-0. P. Nonmetallic element of atomic number 15, group VA of periodic table, aw 30.97376, valences of 1,3,4,5 allotropes white (or yellow), red, and black phosphorus. No stable isotopes, several artificial radioactive isotopes with mass numbers 29-34. 

Black phosphorus, which is one of the crystalline modifications of elemental phosphorus, has a crystal structure of three-dimensional network as shown in Figure 2a (45). A plane diagram of its structure is illustrated in Figure 2b-. On the other hand, various structures have been proposed for commercial red phosphorus, which is one of the amorphous modifications of elemental phosphorus. The fact that s... 

The chemical differences between nitrogen and phosphorus are apparent in their elemental forms. In contrast to the diatomic form of elemental nitrogen, which is stabilized by strong tt bonds, there are several solid forms of phosphorus that all contain aggregates of atoms. White phosphorus, which contains discrete tetrahedral P4 molecules [see Fig. 20.18(a)], is very reactive it bursts into flames on contact with air (it is said to be pyrophoric). Consequently, white phosphorus is commonly stored under water. White phosphorus is quite toxic the P4 molecules are very damaging to tissue, particularly the cartilage and bones of the nose and jaw. The much less reactive forms, called black phosphorus and red phosphorus, are network solids (see Section 10.5). Black phosphorus... 

In Figure 2.6, we show the 3D model of white phosphorus (P4). It is a beautiful cage molecule White phosphorus is the most common structure of the element in nature. Since P is a triple connector, it can make many other molecules which contain only P atoms. Thus, we have also red and black phosphorus, which are much more complex molecules than P4. 

Phosphanes, arsanes, and stilbanes, i.e., hydrogen compounds of phosphorus, arsenic, and antimony, are known in the form of short isochains or rings. The elements phosphorus, arsenic, and antimony exist polymerically with different chain atom arrangements in what are known as their allotropic modifications. The best known example is black phosphorus. 

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