Allotropes of tin

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. 

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. 

The Allotropes 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... 

The most common allotrope of tin is a silver-white metallic-looking solid known as the /3-form (or beta-form ). Allotropes are forms of an... 

Gray tin (the a-form) crystallizes with unit cell dimensions of a = b = c=6.489 A and is a brittle solid that falls apart easily. Urban legend has it that the white tin buttons on Napoleon s soldiers converted to the gray form during his winter invasion of Russia, causing them to fall apart in the cold. Using the crystal structures of each allotrope shown here, determine the density of both allotropes of tin. 

Bond type transformations are characterized by a change in the type of interatomic bonding. Examples are the transitions between allotropes of tin and carbon. These transformations, like reconstructive transitions, are usually very sluggish. 

Van t Hoff was succeeded in Utrecht by Ernst Julius Cohen (Amsterdam, 7 March 1869-Auschwitz, 5 March 1944), who worked on allotropes of tin and antimony, metastability, electrochemistry and piezochemistry, and wrote on the history of chemistry. ... 

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

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. 

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 . 

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. 

A discussion of the nonintegral value, 0.72, of metallic orbitals per atom will be given in the following section, in connection with the discussion of interatomic distances in the allotropic forms of tin. 

III.18 TIN, Sn (Ar 118-69) - Tin(II) Tin is a silver-white metal which is malleable and ductile at ordinary temperatures, but at low temperatures it becomes brittle due to transformation into a different allotropic modification. It melts at 231-8°C. The metal dissolves slowly in dilute hydrochloric and sulphuric acid with the formation of tin(II) (stannous) salts ... 

FIGURE 11.10. The two allotropes (polymorphs) of tin. Grey tin is the low-temperature form, (a) View down a unit cell axis of grey tin (left) and white tin (right), (b) Stereoview of grey tin and (c) stereoview of white tin. In (b) and (c) the surroundings of one tin atom are indicated by solid lines. Other Sn- Sn distances are shown by broken lines. Small circles indicate the corners of the unit cell. 

In the late 19th cenmiy, organ pipes in many cathedrals of Northern Europe were made of tin alloys. During the coldest winters, these pipes began to crumble as tin changed from one allotropic form to the other. The change was known as tin disease. At the time, no one knew why this change occurred. 

The occurrence of polymorphic forms and the persistence of the metastable state are facts of the highest practical and theoretical importance. In the case not only of tin, but also a number of other metals, e.g. bismuth, cadmium, copper, silver, and zinc, allotropic modifications exist with transition points at temperatures above the ordinary and, owing to the slowness of transformation, these metals exist, at the ordinary temperature, in a metastable state. On this fact depends the practical, everyday use of these metals. ... 

In Group 14, only carbon and tin exist as allotropes under normal conditions. For most of recorded history, the only known allotropes of carbon were diamond and graphite. Both are polymeric solids. Diamond forms hard, clear, colorless crystals, and was the first element to have its structure determined by x-ray diffraction. It has the highest melting point and is the hardest of the naturally occurring solids. Graphite, the most thermodynamically stable form of carbon, is a dark gray, waxy solid, used extensively as a lubricant. It also comprises the lead in pencils. 

Elements. Those elements that form extended covalent (as opposed to metallic) arrays are boron, all the Group IV elements except lead, also phosphorus, arsenic, selenium and tellurium. All other elements form either only metallic phases or only molecular ones. Some of the above elements, of course, have allotropes of metallic or molecular type in addition to the phase or phases that are extended covalent arrays. For example, tin has a metallic allotrope (white tin) in addition to that with the diamond structure (grey tin), and selenium forms two molecular allotropes containing Se8 rings, isostruc-... 

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