Allotropes, tin

Figure 5.3 shows the phase diagram of tin, and clearly shows the transition from tin(white) to tin(grey). Unfortunately, the tin allotropes have very different densities p, so p(tm, grey) = 5.8 gem-3 but p(tm, white) = 7.3 g cm-3. The difference in p during the transition from white to grey tin causes such an unbearable mechanical stress that the metal often cracks and turns to dust - a phenomenon sometimes called tin disease or tin pest . 

Below 13 °C, powdery grey a-tin forms on the white, metallic (J-tin allotrope and the metal crumbles. Napoleon s soldiers had tin buttons fastening their jackets and they used tin pots and pans to cook with. During the winter invasion of Russia, in 1812, their buttons and pots crumbled and it was said that this contributed to their defeat. 

There is, in fact, no clear-cut distinction between metals and non-metals. In the periodic table, there is a change from metallic to non-metallic properties across the table, and an increase in metalUc properties down a group. Consequently there is a diagonal around the center of the table (B, Si, As, Te) in which there is a borderline between metals and non-metals, and the metalloids are the borderline cases. Elements such as arsenic, germanium, and tellurium are semiconductors, but other elements are often said to be metalloids according to their chemical properties. Tin, for instance, forms salts with acids but also forms stan-nates with alkalis. Its oxide is amphoteric. Note also that tin has metallic (white tin) and non-metallic (gray tin) allotropes. 

Tin has two allotropes at room temperature the stable modification is white, tetragonal... 

The normal crystal form of tin is body-centred tetragonal, but a low-temperature allotrope, grey tin , is cubic. The transformation temperature... 

Solid allotropes may differ from one another in their bonding patterns (recall Figure 9.11, page 241). White tin, the stable form of solid tin at 25°C, shows metallic bonding... 

Self-Test 7.1 IB Use the information in Table 7.3 or Appendix 2A to determine which allotrope is the more ordered form and predict the sign of AS for each transition (a) white tin (Fig. 7.14) changes into gray tin at 25°C (b) diamond changes into graphite at 25°C. 

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. 

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. 

The Aiiotropes of Tin Tin Pest. Metallic tin may occur in three allotropic forms (see Textbox 19) the common form of tin, also known as white tin or beta tin, is stable at ambient temperatures its stability extends between -18°C and 170°C below -18°C tin is converted to a gray powdery allotrope, known as alpha tin or tin pest. A third allotrope, known as rhombic tin, is the form of tin stable at temperatures above 170°C. If ordinary white tin remains for extended periods of time at temperatures below -18°C, therefore, it is slowly converted to the gray, brittle, and powdery allotrope tin pest the conversion is accelerated at still lower temperatures. Tin objects kept in regions of the world where extremely low temperatures (below -18°C) prevail, initially... 

At atmospheric pressure, pure solid tin adopts two structures or allotropes, depending on temperature. At room temperature white metallic tin is stable but, at temperatures below 13°C, white tin undergoes a phase transformation into gray tin. White tin (also known as / -tin) adopts a body-centered tetragonal crystal structure (Fig. 8.5.1). Allotropic gray tin (a-tin) crystallizes in a cubic diamond... 

Each atom is connected to its neighbors by four bonds pointing toward the vertices of a tetrahedron. The structure can also be considered to be made up of carbon tetrahedra, each containing a central carbon atom. Two other members of group 14, Si and Ge, as well as the allotrope of tin stable below 13.2°C, gray tin or a-Sn, also adopt the diamond structure. 

Metallic tin has many allotropic forms rhombic white tin (also called /3-tin) is stable at temperatures above 13 °C, whereas the stable form at lower temperatures is cubic grey tin (also called a-tin). A transition such as tin(white) tin ey) is called a solid-state phase transition. 

Allosteric coupling, 16 788-789 Allosteric effect, 20 829, 830 Allosteric site, 10 255 Allotropes sulfur, 23 564 tin, 24 786 titanium, 24 838 Alloy 20, 23 784 Alloy 42, 27 841 Alloy broadening, 24 836, 837 Alloy C-276, 23 784 Alloying. See also Alloys... 

Time-weighted average (TWA), 74 215 concentration, 25 372 exposure limit, for tantalum, 24 334 Time-Zero SX-70 film, 79 303, 305-307 Tin (Sn). See Lead-antimony-tin alloys Lead- calcium-tin alloys Lead-lithium-tin alloys Lead-tin alloys, 24 782-800. See also Tin alloys Tin compounds allotropes of, 24 786 analytical methods for, 24 790-792 in antimony alloys, 3 52t atomic structure of, 22 232 in barium alloys, 3 344, 4 12t bismuth recovery from concentrates, 4 5-6... 

Tin exists in three different forms (allotropes). Grey tin has a diamond structure, a density of 5.75 gem-3 and is stable below 286 K. White tin exists as tetragonal crystals, has a density of 7.31 gem-3 and is stable between 286 and 434 K. Between 434 K and the melting point of tin, 505 K, tin has a rhombic structure, hence the name rhombic tin , and a density of 6.56gem-3. 

There are two allotropes of tin. One is known as gray or alpha (a) tin, which is not very stable. The other is known as white tin or beta ((3), which is the most common allotrope. The two forms (allotropes) of tin are dependent on temperature and crystalline structure. White tin is stable at about 13.2°C. Below this temperature, it turns into the unstable gray alpha form. There is also a lesser-known third allotrope of tin called brittle tin, which exists above 161°C. Its name is derived from its main property. 

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. 

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