Germanium atomic properties

Germanium, Tin and Lead Table 10.1 Atomic properties of Group 14 elements... 

The dilithium salt of the tetraphenylgermole dianion has the very interesting property of crystallizing from dioxane in two structurally distinct forms (a and b) depending upon the crystallization temperature. The crystals obtained from dioxane at — 20 °C have a reverse sandwich structure (a), while crystals obtained at 25 °C have one lithium atom / -coordinated to the ring atoms and the other rj1 -coordinated to the germanium atom (b)35 (Figure 8). 

Compounds la-7a and lb-7b are characterized by the presence of a pentacoordinate (formally negatively charged) silicon or germanium atom and a tetracoordinate (formally positively charged) nitrogen atom. Syntheses and properties of the Si/Ge analogues la/lb-7a/7b are reported. In addition, the crystal structures of the X, Ge-germanates 4b and 5b-H20 are described. 

Notice that germanium, Ge, is a hietalloid but tin, Sn, is a metal. What changes in atomic properties do you think are important in explaining this difference ... 

Superatoms are atomic clusters which mimic the behavior of an atom in its elemental state. For example Aly, AI13, and Alj4 exhibit the properties of a germanium atom, a halogen atom, a noble gas atom and an alkaline earth metal atom, respectively. Since Aly is a multivalent superatom it can give rise to stable compounds like AlyC" and AlvO" (Fig. 10) which in turn show the characteristics of SiC and CO, respectively. 

The oxidation state -1-4 is predominantly covalent and the stability of compounds with this oxidation state generally decreases with increasing atomic size (Figure 8.1). It is the most stable oxidation state for silicon, germanium and tin, but for lead the oxidation state +4 is found to be less stable than oxidation state +2 and hence lead(IV) compounds have oxidising properties (for example, see p. 194). 

All Group IV elements form tetrachlorides, MX4, which are predominantly tetrahedral and covalent. Germanium, tin and lead also form dichlorides, these becoming increasingly ionic in character as the atomic weight of the Group IV element increases and the element becomes more metallic. Carbon and silicon form catenated halides which have properties similar to their tetrahalides. 

Gallium [7440-55-3] atomic number 31, was discovered through a study of its spectral properties in 1875 by P. E. Lecoq de Boisbaudran and named from Gallia in honor of its discoverer s homeland. The first element to be discovered after the pubHcation of Mendeleev s Periodic Table, its discovery constituted a confirmation of the Table which was reinforced shordy after by the discoveries of scandium and germanium. 

The technology of silicon and germanium production has developed rapidly, and knowledge of die self-diffusion properties of diese elements, and of impurity atoms has become reasonably accurate despite die experimental difficulties associated widi die measurements. These arise from die chemical affinity of diese elements for oxygen, and from die low values of die diffusion coefficients. 

Use Appendix 2D to find the values for the atomic radii of germanium and antimony as well as the ionic radii for Ge2+ and Sb3+. What do these values suggest about the chemical properties of these two ions ... 

Mendeleev also predicted the existence of elements that had not yet been discovered. His arrangement of the then-known elements left some obvious holes in the periodic table. For instance, between zinc (combines with 2 Cl) and arsenic (combines with 5 Cl) were holes for one element that would combine with three chlorine atoms and another that would combine with four. Mendeleev assigned these holes to two new elements. He predicted that one element would have a molar mass of 68 g/mol and chemical properties like those of aluminum, while the other would have a molar mass of 72 g /mol and chemical properties similar to silicon. These elements, gallium (Z = 31, M M = 69.7 g/mol) and germanium (Z = 32, M M — 72.6 g/mol), were discovered within 15 years. Chemists soon verified that gallium resembles aluminum in its chemishy, while germanium resembles silicon, just as Mendeleev had predicted. 

Qualitatively, we can understand this variation by recalling that as the principal quantum number increases, the valence orbitals become less stable. In tin, the four n — 5 valence electrons are bound relatively loosely to the atom, resulting in the metallic properties associated with electrons that are easily removed, hi carbon, the four n — 2 valence electrons are bound relatively tightly to the atom, resulting in nonmetallic behavior. Silicon ( = 3) and germanium (a = 4) fall in between these two extremes. Example describes the elements with five valence electrons. 

As we can see from the last entry in this table, we have deduced only a rule. In InBi there are Bi-Bi contacts and it has metallic properties. Further examples that do not fulfill the rule are LiPb (Pb atoms surrounded only by Li) and K8Ge46. In the latter, all Ge atoms have four covalent bonds they form a wide-meshed framework that encloses the K+ ions (Fig. 16.26, p. 188) the electrons donated by the potassium atoms are not taken over by the germanium, and instead they form a band. In a way, this is a kind of a solid solution, with germanium as solvent for K+ and solvated electrons. K8Ge46 has metallic properties. In the sense of the 8-A rule the metallic electrons can be captured in K8Ga8Ge38, which has the same structure, all the electrons of the potassium are required for the framework, and it is a semiconductor. In spite of the exceptions, the concept has turned out to be very fruitful, especially in the context of understanding the Zintl phases. 

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