Gray tin

Tin exists in two ahotropic forms white tin (P) and gray tin (a). White tin, the form which is most familiar, crystallizes in the body-centered tetragonal system. Gray tin has a diamond cubic stmcture and may be formed when very high purity tin is exposed to temperatures well below zero. The ahotropic transformation is retarded if the tin contains smah amounts of bismuth, antimony, or lead. The spontaneous appearance of gray tin is a rare occurrence because the initiation of transformation requires, in some cases, years of exposure at —40° C. Inoculation with a-tin particles accelerates the transformation. 

Organ pipes in unheated churches develop tin disease, in which white tin is converted to gray tin. Given... 

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. 

Successive pivoting resonances of a covalent bond allows for electrical conduction to occur, as shown in Figure 1-1. A test of this theory was provided by gray and white tin. Gray tin is not metallic because all its valence orbitals are used for bonding and there is no metallic orbital available. White tin, on the other hand, has the metallic orbital available and therefore has metallic properties. 

In this discussion of the transition elements we have considered only the orbitals (n— )d ns np. It seems probable that in some metals use is made also of the nd orbitals in bond formation. In gray tin, with the diamond structure, the four orbitals 5s5p3 are used with four outer electrons in the formation of tetrahedral bonds, the 4d shell being filled with ten electrons. The structure of white tin, in which each atom has six nearest neighbors (four at 3.016A and two at 3.17.5A), becomes reasonable if it is assumed that one of the 4d electrons is promoted to the 5d shell, and that six bonds are formed with use of the orbitals 4dSs5p35d. 

It was pointed out in my 1949 paper (5) that resonance of electron-pair bonds among the bond positions gives energy bands similar to those obtained in the usual band theory by formation of Bloch functions of the atomic orbitals. There is no incompatibility between the two descriptions, which may be described as complementary. It is accordingly to be expected that the 0.72 metallic orbital per atom would make itself clearly visible in the band-theory calculations for the metals from Co to Ge, Rh to Sn, and Pt to Pb for example, the decrease in the number of bonding electrons from 4 for gray tin to 2.56 for white tin should result from these calculations. So far as I know, however, no such interpretation of the band-theory calculations has been reported. 

Pure tin exhibits two common forms in the solid state — a gray tin and a white tin. At temperatures above 13°C or 55°F, the more stable form of tin is the denser white tin. At lower temperatures, the white tin is slowly converted to the gray form, a more powdery substance. Prolonged exposure to the cold winter temperatures of northern Europe contributed to the loss of integrity and disintegration of many cathedral organ pipes. As a consequence of the progressive nature of the structural transformation, as the white tin metallic surface becomes covered with... 

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... 

Figure 8.5.2 A cubic diamond crystal structure adopted by gray 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. 

Compounds in which tin is covalently bonded lie near the center of the diagram—gray tin and several organometallic compounds. Stannic compounds lie in a comparatively narrow group near —2 mm./sec., while stannous compounds lie in a large band to the right of zero. 

FIGURE 7.13 Gray tin and white tin are two solid forms of tin. The denser white metallic form is the more stable phase above 13°C, and the powdery gray form is more stable below that temperature. 

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