Non metallic character

Arsenic exhibits allotropy, which is characteristic of non-metals the usual, more stable, metallic form resembles the typical metals in appearance and in being a fairly good conductor of electricity. Under atmospheric pressure it begins to volatilise at about 450° C. and passes into a vapour containing complex molecules, As4, which at higher temperatures dissociate to As2 this complexity is not unusual in non-metals. The yellow allotrope, which is stable at low temperatures, resembles white phosphorus in being soluble in carbon disulphide—a property which emphasises the non-metallic character of this variety. The reactivity of the allotropes, as in the case of phosphorus, differs considerably. 

Silicides. Crystn compds formed from silicon and various other elements of metallic, transitional or non-metallic character. A complete discussion of silicides is to be found in the work of Sansonov (Ref 9). Those silicides of ordn interest are presented below... 

The unique properties of the stable d5 Mn2+ ion is reflected by the fact that MnS and MnSe crystallize in three modifications, in the rocksalt, the cubic sphalerite and the hexagonal wurtzite structure. While in the NiAs structure of MnTe the cations occupy the octahedral holes of a hexagonal close-packing of anions they occupy half of the tetrahedral holes of this packing in the ZnO type modification of MnS and MnSe. The non-metallic character is evident already from the fact that the structure is undistorted (c/a = 1.61 for MnS and 1.63 for MnSe) and that the cations really are at the centres of one set of tetrahedral holes and not at the centre of the bipyramidal holes composed of two tetrahedra of the two different sets. 

C Atomic Radii Ionisation energy IE of Pb > Sn Inner part effect Electronegativity IE of Ge Si Si Metallic character Non-metallic character Only C has ability of... 

For real metals this value is close to 1. This value shows qualitatively if the association to homo-nuclear two-atomic molecules (AH°diss) (non metallic character) is energetically preferred over the formation of a coordination lattice (metallic character) and vice versa. According to this relation a metallic character can also be expected for the elements 112 and 114 [28]. Element 117, for example, can be assumed to have a semi-metallic character. 

The elements that display the greatest non-metallic character are located toward which corner of the periodic table ... 

Although the metals are most commonly depicted with charge numbers -f-5 or -j-3, their tendency towards non-metallic character is revealed in... 

Oxygen occupies so central a position in any treatment of inorganic chemistry that discussions of many of its compounds are dealt with under other elements. The decrease in non-metallic character down the group is easily recognized in the elements ... 

Compounds containing M—C bonds where M is an 5-block element are readily classified as being organometallic. However, when we come to the p-block, the trend from metallic to non-metallic character means that a discussion of strictly organometallic compounds would ignore... 

The trends in metallic and non-metallic character of the elements within groups and periods within the periodic table may be summarized in the following diagram ... 

From left to right across each period, metallic character gives way to non-metallic character (Figure 3.11). The elements with the most metallic character are at the lower-left part of the periodic table near cesium (Cs). The elements with the most nonmetaUic character are at the upper-right portion of the periodic table near fluorine. The six metalloid elements (in blue) that begin with boron and move down like a staircase to astatine (At) roughly separate the metals and the nonmetals. 

Relative to these deviations, we have also calculated other parameters regarding the Si atoms with the Bader partition scheme of the electron density for both (110) and (100) planes (Table 11.3). The obtained Bader charges for the Si(sp ) or Si(sp ) atoms possess opposite signs for both (110) and (100) planes. The different charges are coherent with the proposed origin of the band gap of 0.5 eV for a partially ionic Si-Si bond [45], which explains the non-metallic character of the Si (100) surface [51]. Small differences are observed between the Si(ip ) Bader charges for the (110) and (100) surface but the Si sp ) charges coincide for both planes. 

Metals frequently form intermetallic compounds, particularly when the individual pure metals have different structures. Intermetallic compounds exhibit a more (Cu-Mg) or less (Cu-Zn) narrow range of homogeneity. Metallic bonding is the predominant type of bonding in many intermetallic compounds. However, a certain fraction of the other types of bonding must also be taken into consideration. This partially non-metallic character of the bonds, as well as the different sizes of the atoms, is responsible for the large variety of crystal structures of intermetallic compounds. Only some of the most important groups will be mentioned here. 

Compounds containing M—C bonds where M is an 5-block element are readily classified as being organometallic. However, when we come to the p-block, the trend from metallic to non-metallic character means that a discussion of strictly organometallic compounds would ignore compounds of the semi-metals and synthetically important organoboron compounds. For the purposes of this chapter, we have broadened the definition of an organometallic compound to include species with B—C, Si C, Ge—C, As—C, Sb—C, Se C or Te—C bonds. Compounds... 

The amphoteric character of aluminum hydroxide is a reflection of the partial non-metallic character of aluminum. Table 22.5 lists the major uses of aluminum compounds. 

Thus metals are grouped on the left-hand side, whereas non-metals are grouped on the right. The most reactive metals are on the left and at the bottom of the periodic table. The most reactive non-metals are on the right and at the top of the periodic table. From left to right across a period, there is a decrease in metallic character and an increase in non-metallic character (Table 3.19). 

Selenium is on the right-hand side and towards the middle of the periodic table. Non-metallic character increases across the periodic table (left to right) and decreases down a group. 

Fig. 27. Formally constructed stability diagram for the lanthanide monosulphides. The diagram is obtained from the Fermi level referenced data for the semiconducting compounds SmS, EuS and YbS (Mirtensson et al. 1982b). Complications due to the non-metallic character of the divalent monosulphides, as for instance incomplete screening, are discussed in the text ...

The values added in brackets refer to an ideal cubic close-packing of Sn atoms. The reduction in symmetry is paraded by a reduction of the number of bonded Sn—O contacts and an enhancement of the non-metallic character. The two-dimensional structure thereby transforms into a molecular structure with ring-shaped Sn202 molecules. This transition is quite analogous to the sequences from three-dimensional to molecular structures observed in compounds with other lone-pair cations such as Bi —> P and Te —> S. The four equivalent Sn—O distances of 2.21 A in tetragonal SnO go over into two sets of pairs in the orthorhombic modification, namely... 

The anion-anion distances, S—S = 2.13A and Se-Se = 2.36A, are slightly larger than expected for covalent single bonds but the non-metallic character of these diamagnetic compounds is a proof that these are single bonds. When the distortion of the anion octahedra in PdS2 is reduced by pressure the semiconductor transforms into a metal before the pyrite structure is reached [419]. 

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