Crystalline solids allotropes

Phosphorus exists as white and red phosphorus. The former allotrope may be preserved in the dark at low temperatures but otherwise reverts to the more stable red form. The white form is a waxy, translucent, crystalline, highly-toxic solid subliming at room temperature and inflaming in air at 35°C, so it is handled under water. The red form is a reddish violet crystalline solid which vaporizes if heated at atmospheric pressure and condenses to give white phosphorus. The red form ignites in air at 260°C. Both are insoluble in water, and white phosphorus can be stored beneath it. Phosphorus forms a host of compounds such as phosphine, tri- and penta-halides, tri-, tetra- and penta-oxides, oxyacids including hypophosphorous, orthophosphorous and orthophosphoric acids. 

Trigonal crystalline solid or amorphous powder mineral millerite has a yellow metallic luster color varies from yellow to brownish black density 5.30 to 6.65 g/cm3 exhibits three allotropic modifications (1) the acid-soluble amorphous alpha form obtained from nickel salt solution by precipitation with ammonium sulfide, (2) the alpha form rapidly transforms to a crystalline beta form as a brown colloidal dispersion upon exposure to air, and (3) a rhombo-hedral gamma modification found native as mineral millerite, which also can be prepared artificially under certain conditions. 

Solid Allotropes.—Both amorphous and crystalline varieties of selenium occur. Amorphous selenium is best known as the vitreous and the finely divided brick-red forms, which are frequently described as two distinct allotropes they are, however, identical. The crystalline allotropes include several monoclinic varieties, red to brown in colour, as well as the so-called metallic selenium. 

Sulfur exists in many allotropic forms, but the most stable at 25°C is rhombic sulfur, a yellow crystalline solid (mp 113°C) that contains crown-shaped S8 rings ... 

In 1991, scientists at AT T Bell Laboratories discovered a new class of high-temperature superconductors based on fullerene, the allotrope of carbon that contains Cgo molecules (Sections 10.10 and 19.6). Called "buckyballs," after the architect R. Buckminster Fuller, these soccer ball-shaped Cgo molecules react with potassium to give K3C6o- This stable crystalline solid contains a face-centered cubic array of buckyballs, with K+ ions in the cavities between the Cgo molecules (Figure 21.16). At room temperature, K3Q,o is a metallic conductor, but it becomes a superconductor at 18 K. The rubidium fulleride, Rb C o, and a rubidium— thallium-Cfio compound of unknown stoichiometry have higher Tc values of 30 K and 45M8 K, respectively. 

Sulfur has three solid allotropes, two different crystalline forms based on small Ss molecules called rhombic and monoclinic sulfur and a 3rd form of long chain (-S-S-S- etc.) molecules called plastic sulfur. 

Arsenic occurs in two allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The more common form of arsenic is a shiny, gray, brittle, metallic-looking solid. The less common form is a yellow crystalline solid. It is produced when vapors of arsenic are cooled suddenly. 

Properties Silver-gray, brittle, crystalline solid that darkens in moist air. Allotropic forms black, amor-... 

The following section provides a brief summary of the molecular and crystal structures of the solid allotropes of sulfur mentioned in the Introduction. More specific details about the structures of most of the allotropes can be found in the cited literature. A conclusion concerning the characteristics of the molecular as well as of the crystalline structures of sulfur will be drawn at the end of this section. 

Up to now, the various solid allotropes of polymeric sulfur can only be identified by X-ray structural analysis since the molecular vibrations and therefore the Raman spectra are very similar [151]. A chance to distinguish between the different allotropes by vibrational spectroscopy may only exist if the lattice vibrations are recorded which are expected to be different enough, especially for the fibrous and the laminar structures. However, this experiment requires samples of a high degree of crystallinity such measurements have to be performed at low temperatures to avoid broadening of the Raman lines. 

When white phosphorus is heated at 200° under a pressure of 12,000 kgm. per sq. cm., transformation takes place into another allotropic modification known as black phosphorus. This forms a black crystalline solid, insoluble in carbon disulphide. It can be ignited with difficulty with a match, its ignition temperature in air being about 400°. When heated in a closed tube it vaporises and condenses to violet and white phosphorus. It differs from the other forms of phosphorus in being a conductor of electricity. Its density is 2 691, The question of the relative stability of violet and black phosphorus has perhaps not yet been definitely settled but the results obtained point to violet phosphorus being the more stable form, ... 

Of the solids given, ionically bonded sodium chloride is expected to be crystalline, a poor electrical conductor in the solid form, and a good conductor when fused. Diamond, formed of covalently bonded carbon atoms, is a network substance that does not form cubic crystalline patterns, and does not conduct electricity either when solid or fused. None of the allotropic forms of sulfur is expected to conduct electricity. Choice (D), the metal chromium, could possibly form a cubic solid crystalline form, but can be eliminated because it is expected to conduct electricity both when a solid and when fused. The correct choice is (A), because sodium chloride is a crystalline solid that is a poor conductor in the solid state and a good conductor when fused. ... 

The first statement that an allotropic form of the element is a colorless crystalline solid, might lead you to think about diamond, a form of carbon. When carbon is reacted with excess oxygen, the colorless gas, carbon dioxide is produced. 

Carbon occurs in several different bonding patterns, called allotropes, which have very different properties. Diamond is a colorless, crystalline solid form of carbon, in which each atom is tetrahedrally bonded to four others in a network fashion. This three-dimensional bonding makes diamond the hardest material known. 

One allotropic form of an element X is a colorless crystalline solid. The reaction of X with an excess amount of oxygen produces a colorless gas. This gas dissolves in water to yield an acidic solution. Choose one of the following elements that matches X (a) sulfur, (b) phosphorus, fc) carbon, (d) boron,... 

In a few substances, known as network covalent solids, covalent bonds extend throughout a crystalline solid. In these cases, the entire crystal is held together by strong forces. Consider, for example, two of the allotropic forms in which pure carbon occurs—diamond and graphite. 

Entropy of Methylammonium Chloride. Heat capacities for this solid in its various crystalline modifications have been determined [10] precisely down to 12 K. Some of these data are summarized in Figure 11.3. There are three crystalline forms between OK and 298K. One can calculate the entropy by integrating Equation (11.21) for each allotrope in the temperature region in which it is most... 

Crystalline samples are produced by stretching polymeric sulfur during or after chilling (25). Solid samples contain always other sulfur allotropes, among them rings. It is now believed that the long sulfur helices contain 10 atoms for every three turns. The best presently available bond data (25) are... 

Many elements can give rise to more than one elementary substance. These may be substances containing assemblages of the same mono- or poly-atomic unit but arranged differently in the solid state (as with tin), or they may be assemblages of different polyatomic units (as with carbon, which forms diamond, graphite and the fullerenes, and with sulfur and oxygen). These different forms of the element are referred to as allotropes. Their common nomenclature is essentially trivial, but attempts have been made to develop systematic nomenclatures, especially for crystalline materials. These attempts are not wholly satisfactory. 

Steel-gray crystalline brittle metal hexagonal crystal system atomic volume 13.09 cc/g atom three allotropes are known namely, the a-metaUic form, a black amorphous vitreous solid known as P-arsenic, and also a yellow aUotrope. A few other allotropes may also exist but are not confirmed. Sublimes at 613°C when heated at normal atmospheric pressure melts at 817°C at 28 atm density 5.72 g/cc (P-metallic form) and 4.70 g/cm (p-amor-phous form) hardness 3.5 Mohs electrical resistivity (ohm-cm at 20°C) 33.3xlCh (B—metallic polycrystalline form) and 107 (p—amorphous form) insoluble in water. 

By rapidly cooling the fluid mixture it is possible to minimise the readjustment of the equilibrium and to attain a solid condition in which tire original proportions of the mixture are approximately retained in the solid state tire allotropic change is so very slow as to allow careful and fairly prolonged examination of the mixture. It is then found that, the normal mobile liquid constituent (S ) has given rise to crystalline sulphur, soluble in carbon disulphide, whereas the dark-coloured viscous constituent (S ) has produced an amorphous solid, insoluble in this solvent4 (see also p. 10). A rough analysis of molten sulphur in... 

Carbon exists in more than 40 known structural forms, or allotropes, several of which are crystalline but most of which are amorphous. Graphite, the most common allotrope of carbon and the most stable under normal conditions, is a crystalline covalent network solid that consists of two-dimensional sheets of fused six-membered rings (Figure 10.26a). Each carbon atom is sp2-hybridized and is connected to three other carbons. The diamond form of elemental carbon is a covalent network solid in which each carbon atom is sp3-hybridized and is bonded with tetrahedral geometry to four other carbons (Figure 10.26b). 

We have discussed the structure of crystalline N2 in Section 4.4.1. Several allotropes of phosphorus are well known. In the gas phase there are P4 tetrahedral molecules, and this condenses as white (also described as yellow) P containing P4 molecules in the solid. There are two forms of white P, but the detailed structures are not known. There are inconsistencies in the structural reports. A monoclinic form of P contains cage-like P,s and P9 groups linked by pairs of P atoms to form... 

The various crystalline forms in which a solid may exist are called allotropes or polymorphs. The transformation from one allotrope to another is called allotropic or polymorphic transformation. This transformation may occur either with pressure change or with temperature change. 

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. 

Allotropic forms of sulfur. Solid sulfur exists in two crystalline modifications. Rhombic sulfur consists of S8 molecules, is stable at temperatures below 95.5°C, has a specific gravity of 1.96, and is soluble in carbon disulfide. At 95.5°C, rhombic sulfur changes slowly, with absorption of heat, into the monoclinic form. Molten rhombic sulfur consists of S8 molecules and exists as a pale-yellow, thin, and limpid liquid known as X sulfur. When the temperature is raised, X sulfur is slowly converted to dark and viscous p sulfur, which consists of Ss and S4 molecules and which is considered to be the amorphous variety of... 

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