Allotropy

Allotropy.—Dimorphism apart, a few substances are known to exist in more than one solid form. These varieties of the same substance exhibit different physical properties, while their chemical qualities are the same in kind. Such modifications are said to be allotropic. One or more allotropic modifications of a substance are usually crystalline, the other or others amorphous or vitreous. Sulfur, for example, exists not only in two dimor-j)hous varieties of crystals, but also in a third,. allotropic form, in which it is flexible, amorphous, and transparent. Carbon exists in three allotropic forms two crystalline, the diamond and graphite the third amorphous. 

In passing from one allotropic modification to another, a substance absorbs or gives out heat. 

Preparation of Red Phosphorus. (This experiment is prepared by one student who demonstrates it to the class.) Take a test tube with a drawn out end and fill it with carbon dioxide gas. Pour water heated 

Heat the drawn out part of the test tube until the glass melts, shape its end into a hook, and seal the tube. Spill 8-10 g of naphthalene onto the bottom of a wide bulbshaped test tube at least 40 cm long. Lower into it the sealed test tube prepa- Containing the phosphorus and fasten the hook-shaped end of the tube with copper wire to a glass rod (Fig. 93). Cover the mouth of the wide test tube with a cardboard disk. 

Fasten the wide tube in a stand and carefully heat it through asbestos gauze during three or four hours, seeing that the ampoule is constantly in naphthalene vapour. 

Extract the sealed tube and see how the colour of the phosphorus changed. 

Preparation of White Phosphorus. Put a small amount of dry red phosphorus on the bottom of a test tube and close its mouth with a piece of cotton wool. Secure the tube in a clamp of a stand and carefully heat the place where the phosphorus is (in a fume cupboard ). What happens Observe the luminescence of the phosphorus in the dark. 

The a- and 5-forms of red crystalline Scg are obtained respectively by the slow and rapid evaporation of CS2 or benzene solutions of black vitreous Se and more recently a third (y) form of red crystalline Scg was obtained from the reaction 

Tellurium has only one crystalline form and this is composed of a network of spiral chains similar to those in hexagonal Se (Fig. 16.1c and d). Although the intra-chain Te-Te distance of 284 pm and the c dimension of the crystal (593 pm) are both substantially greater than for Scjt (as expected), nevertheless the closest interatomic distance between chains is almost identical for the 2 elements (Te Te 350 pm). Accordingly the elements form a continuous range of solid solutions in which there is a random 

Preparation of Red Phosphorus. (This experiment is prepared by one student who demonstrates it to the class.) Take a test tube with a drawn out end and fill it with carbon dioxide gas. Pour water heated to 30 °C into a mortar and lower a piece of white phosphorus into the water. Holding the phosphorus with pincers under the water, cut off a piece the size of a pea with a lancet or sharp knife. Rapidly dry the phosphorus with filter paper on a glass plate and lower it into the test tube filled with car- 

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The eommonest erystalline forms of earbon, cubie diamond and hexagonal graphite, are elassical examples of allotropy that are found in every chemistry textbook. Both diamond and graphite also exist in two minor crystallographie forms hexagonal diamond and rhombohedral graphite. To these must be added earbynes and Fullerenes, both of which are crystalline earbon forms. Fullerenes are sometimes referred to as the third allotrope of carbon. However, sinee Fullerenes were diseovered more recently than earbynes, they are... 

Allotropy is a very common phenomenon shown by metals, metalloids, and nonmetals. In the gaseous or liquid state, allotropes most often differ from one another in molecular formula. Consider, for example, the two allotropes of gaseous oxygen, 02, and ozone, 03. 

Allotropy in the solid state can also arise because of differences in crystal structure. For example, solid iron has a body-centered cubic structure (recall Figure 9.16, page 246) at room temperature. This changes to a face-centered structure upon heating to 910°C. 

By the term state we refer to such data as the chemical composition of the parts of the system (including allotropy and isomerism), their state of electrification, magnetisation, stress or strain, their state of division, temperature, and the like, and the second condition generalises the statement of equilibrium. 

F. Tuinstra, Structural Aspects of the Allotropy of Sulfur and the Other Divalent Elements, Waltman, Delft, 1967... 

A number of chemical elements, mainly oxygen and carbon but also others, such as tin, phosphorus, and sulfur, occur naturally in more than one form. The various forms differ from one another in their physical properties and also, less frequently, in some of their chemical properties. The characteristic of some elements to exist in two or more modifications is known as allotropy, and the different modifications of each element are known as its allotropes. The phenomenon of allotropy is generally attributed to dissimilarities in the way the component atoms bond to each other in each allotrope either variation in the number of atoms bonded to form a molecule, as in the allotropes oxygen and ozone, or to differences in the crystal structure of solids such as graphite and diamond, the allotropes of carbon. 

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

Another element that exhibits allotropy because of variations in the crystal structure is tin. The common allotrope is tin metal, also known as a alpha) tin, which is stable at ambient temperatures. The other allotrope, which generally occurs as a gray powder and is known as p beta) tin, but also as tin pest, is formed only at very low temperatures when tin cools down to temperatures below -18°C, the ordinary allotrope, a tin, is converted to p tin, and the transformation is irreversible under ordinary temperatures. Tin objects exposed to temperatures below -18°C in very cold regions of the world, for example, are generally severely damaged when part of the tin converts to tin pest. In extreme cases, when exposure to low temperatures extends for long periods of time, the allotropic conversion may result in the transformation of tin objects into heaps of gray p-tin powder. 

All five of the Group VI elements have a property called allotropy, which means they exist in more than one solid form. Like carbon, which could be black graphite or a sparkling diamond, these elements can exist with their atoms arranged in more than one way. 

Many elements including sulphur, carbon and oxygen can exist in two or more forms with different physical, and often chemical, properties such elements are said to exhibit allotropy and the different forms are known as allotropes or allotropic forms. 

An allotropic form of oxygen (03) (see Allotropy) of considerably greater degradative effect on rubber than oxygen itself. The ozone content of the atmosphere normally varies from 0.5 to 5.0 parts per 100 million (pphm) of air, but in certain areas (notably Los Angeles, USA) it may reach as much as 40 pphm of air. 

Dye wastewater, 9 431 Dynamic affinity chromatography, 6 398 Dynamic allotropy, 23 564 Dynamically formed membranes, 15 813t Dynamic coefficient, 15 205 Dynamic compressors, in refrigeration systems, 21 535... 

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