Non-metals properties

In this formula, which can only be applied if all bonds are two-electron bonds and additional electrons remain inactive in non-bonding orbitals (or, in other words, if the compound is semiconductor and has non-metallic properties), ecc is the average number of valence electrons per cation which remain with the cation either in nonbonding orbitals or (in polycationic valence compounds) in cation—cation bonds similarly cAA can be assumed to be the average number of anion—anion electron-pair bonds per anion (in polyanionic valence compounds). 

Periodic trends in ionization energy are linked to trends involving the reactivity of metals. In general, the chemical reactivity of metals increases down a group and decreases across a period. These trends, as well as a further trend from metallic to non-metallic properties across a period, and increasing metallic properties down a group, are shown in Table 3.1. 

Small aggregates of silver (D < 2 nm) having sharp absorption bands in the 300-350 nm region and non-metallic properties. 

In the fourth group, carbon and silicon are both non-metallic, while germanium has a very small electrical conductivity. It is only with white tin and lead that the electrical conductivity approaches the normal values for true metals. In the fifth group, arsenic and antimony are just on the limit between metallic and non-metallic properties, while of the elements of the sixth group, only polonium might be considered to have real metallic properties. The halogens, in the seventh group, show no trace of metallic properties. 

This formula thus represents a crystallographic version of the condition for non-metallic properties. As an example we may check the case of pyrite FeS2 4(6—0) =8(4—1). However, the isostructural C0S2 and CuS2 are metallic though they fulfil equation (6) as well. If we had used the "chemical equation (3) or (4), we would have immediately met the... 

Another distorted variant of the NiAs structure occurs in NiP which is stable only above 850° C (159). In the orthorhombic NiP structure the distortions are stronger than in the MnP type (Fig. 44) but like in MnP the metal atoms form zig-zag chains with Ni—Ni = 2.53 A. The coordination of the nickel atoms is modified insofar as they are shifted towards a comer of the distorted anion octahedra. As a result there are only five phosphorus atoms in contact with the central nickel atom. The anions themselves are arranged in pairs with a P—P distance of 2.43 A, which roughly corresponds to the length of a half bond. In the absence of cation-cation bonds the P—P pairs would lead to divalent Ni and non-metallic properties would be possible. In the actual structure the Ni—Ni bonds exclude semiconductivity which, moreover, cannot be expected in a high-temperature phase. 

An ordered cation-deficient derivative of the NaCl structure was found in the orthorhombic SC2S3 type (168). The unit cell contains twelve rocksalt units (a = 2 ao, b = ]/2 ao, c = 3J/2 o) The cation vacancies are three-dimensionally distributed without pronounced directional preference. In each sulfur-centered octahedron two scandium atoms are missing. SC2S3, ScaSe3, Y2S3 and Y2Se3 (213) Eire normal valence compounds with non-metallic properties. 

The rickardite structure was found also in Ni8 zTe2 (218), In the nickel compound non-metallic properties are excluded already by equation (7). Above 140—250° C the occupation of the octahedral sites is random but at lower temperatures the cations order and a monoclinic superstructure is formed on the nickel-rich side while a tetragonal cell results on the tellurium-rich side of the composition range (218). 

The metallic properties are most pronounced for elements in the lower left-hand corner of the periodic table, and the non-metallic properties are most pronounced for elements in the upper right-hand corner. The transition from metals to non-metals is marked by the elements with intermediate properties, which occupy a diagonal region extending from a point near the upper center to the lower right-hand corner. These elements, which are called metalloids, include boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.. 

E13.27 Statement (a) is incorrect—in fact only boron has some non-metallic properties, all other elements are metallic. Statement (b) is also incorrect— the hardness is decreasing down the group in fact, of Group 13 elements only boron and aluminium (top of the group) show pronounced oxophilicity and boron fluorophilicity. 

Arsenic is an ubiquitous element showing metallic and non-metallic properties with... 

The heavy black line in Fig. 20.1 separates the metals from the nonmetals, except for one case. Flydrogen, which appears on the metal side, is a nonmetal. Some elementsjust on either side of this line, such as silicon and germanium, exhibit both metallic and non-metallic properties. These elements are often called metalloids, or semimetals. The fundamental chemical difference between metals and nonmetals is that metals tend to lose their valence electrons to form cations, which usually have the valence electron configuration of the noble gas from the preceding period. On the other hand, nonmetals tend to gain electrons to form anions that exhibit the electron configuration of the noble gas in... 

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. 

Fig. IV.4. Charge of lead sulfide particles on copper (a) or cadmium (b) substrate as a function of potential impressed on substrate (1) natural powder (2) powder with metallic properties after vacuum treatment (3) powder with non-metallic properties after treatment in sulfur vapor.

Based on the structural data one would expect non-metallic properties but the silvery white crystals are reported to show metallic conductivity [424]. 

In the Re and Tc dichalcogenides there are three excess d electrons that must be localized in order to produce non-metallic properties. These compounds are diamagnetic semiconductors with energy gaps near 1 eV. Thus, the cation clustering is evidently sufficient to secure non-metallic properties. 

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