Germanium allotropic forms

Silicon and germanium are metalloids, whereas tin and lead are metals. Silicon and germanium have structures similar to that of diamond, tin exists in allotropic forms, but lead exists in only one metallic form. 

Silicon and germanium as elemental substances are found only in the diamond-type form. The reluctance of Si and Ge to enter into pre-p bonding prohibits a graphite-type structure as a plausible allotrope. These are rather more reactive than diamond the weaker Si-Si and Ge-Ge bonds make disruption of the lattice kinetically easier. Tin occurs in both a metallic form (white tin) and a covalent (diamond-type) form the latter is slightly more stable at low temperatures. Lead forms only a metallic elemental substance. 

Elements along a rough diagonal from boron to polonium are intermediate in behavior, in some cases having both metallic and nonmetallic allotropes (elemental forms) these elements are designated as metalloids or semimetals. As described in Chapter 7, some elements, such as silicon and germanium, are capable of having their conductivity finely tuned by the addition of small amounts of impurities and are consequently of enormous importance in the manufacture of semiconductors in the computer industry. 

Carbonates and reduced forms of carbon are common on Earth, and silicates make up the major part of the crust germanium is much less common. All elements can form the diamond structure graphite and other allotropes are unique to carbon. 

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. 

As noted earlier, metallic character increases in moving down a group of elements. Tin, which is between the metalloid germanium and the metal lead in Group IVA, illustrates this periodic trend in a very interesting way it has two different forms, or allotropes one is a metal and the other is a nonmetal. 

Interestingly, zinc sulfide (p-ZnS) may also crystallize in a cubic lattice, which consists of a fee array of S , with Zn occupying 1/2 of the available tetrahedral sites. This structure is known as sphalerite or zincblende, and is shared with other compounds such as a-AgI, p-BN, CuBr, and p-CdS. When the same atom occupies both the fee and tetrahedral interstitials of the sphalerite structure, it is described as the diamond lattice, shared with elemental forms (allotropes) of silicon, germanium, and tin, as well as alloys thereof. Important semiconductors such as GaAs, p-SiC, and InSb also adopt the sphalerite crystal structure. 

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