Melting block metals

The interstitial carbides are compounds formed by the direct reaction of a d-block metal and carbon at temperatures above 2000°C. In these compounds, the C atoms occupy the gaps between the metal atoms, as do the H atoms in metallic hydrides (see Fig. 14.9). Here, however, the C atoms pin the metal atoms together into a rigid structure, resulting in very hard substances with melting points often well above 3000°C. Tungsten carbide, WC, is used for the cutting surfaces of drills, and iron carbide, FesC, is an important component of steel. 

Periodic variations in the surface tension of liquid metals, c1 , are shown in Figure 6.5. The much higher surface tension of rf-block metals compared to the s- and p-block metals suggests that the surface tension relates to the strength of interatomic bonding. Similar periodic trends can be found also for the melting temperature and the enthalpy of vaporization, and the surface tension of liquid metals is strongly... 

If granulated tin is not available it may be prepared by melting block-tin over a blow-pipe in a long-handled iron spoon provided with a spout, and then pouring the metal drop by drop from a height of 2-3 feet into a bucket filled with water. 

I nsoluble oxides with high melting points formed from elements in the left of the p block Oxides with low melting points, often gaseous formed from elements on the right of the p block Metallic elements and metalloids... 

Nitrides of the J-block metals are hard, inert solids which resemble metals in appearance, and have high melting points and electrical conductivities (see Box 14.5). They can be prepared from the metal or metal hydride with N2 or NH3 at high temperatures. Most possess structures in which the nitrogen atoms occupy octahedral holes in a close-packed metal lattice. Full occupancy of these holes leads to the stoichiometry MN (e.g. TiN, ZrN, HfN, VN, NbN) cubic close-packing of the metal atoms and an NaCl lattice for the nitride MN is favoured for metals in the earhest groups of the J-block. 

Most elements combine with phosphorus to give binary phosphides exceptions include Hg, Pb, Sb, Bi and Te. Types of solid state phosphides are very varied, and simple classification is not possible. Phosphides of the d-block metals tend to be inert, metallic-looking compounds with high melting points and electrical conductivities. Their formulae are often deceptive in terms of the oxidation state of the metal and their structures may contain isolated P centres, P2 groups, or rings, chains or layers of P atoms. 

Figure 2.22 Cell used for high-melting alkali metal fluorides (A) tube with screw cap, (B) brass head, (C) Viton O-ring, (D) cooling jacket, (E) Pyrex glass support tube, (F) cell body made from nickel or Inconel, (G) stainless-steel conductor block, (H) graphite or tungsten cathode, (I) graphite anode, (M) stainless steel tube, (N) Pyrex glass gas inlet mbe, (O) ball joint, (P) vacuum valves (37).

The stmctures of the so-called interstitial carbides (formed by heating C with fi -block metals having > 130 pm, e.g. Ti, Zr, V, Mo, W) may be described in terms of a close-packed metal lattice with C atoms occupying octahedral holes (see Fig. 6.5). In carbides of type M2C (e.g. V2C, Nb2C) the metal atoms are in an hep lattice and half of the octahedral sites are occupied. In the MC type (e.g. TiC and WC), the metal atoms adopt a cep stmcture and all the octahedral holes are occupied. These interstitial carbides are important refractory materials they are very hard and infusible, have melting points >2800 K and, in contrast to the acetylide... 

Table 4.18 Melting and boiling points of s-block metals... 

The d-block metals (except mercury and cadmium) typically have relatively high melting and boiling points (Figure 13.6) compared to the non-d-block metals.This is a consequence of strong metallic bonding (Chapter 4) because the first-row d-block metals have valence electrons from the 3d and 4d sub-levels. 

Production and Economic Aspects. Thallium is obtained commercially as a by-product in the roasting of zinc, copper, and lead ores. The thallium is collected in the flue dust in the form of oxide or sulfate with other by-product metals, eg, cadmium, indium, germanium, selenium, and tellurium. The thallium content of the flue dust is low and further enrichment steps are required. If the thallium compounds present are soluble, ie, as oxides or sulfates, direct leaching with water or dilute acid separates them from the other insoluble metals. Otherwise, the thallium compound is solubilized with oxidizing roasts, by sulfatization, or by treatment with alkaU. The thallium precipitates from these solutions as thaUium(I) chloride [7791 -12-0]. Electrolysis of the thaUium(I) sulfate [7446-18-6] solution affords thallium metal in high purity (5,6). The sulfate solution must be acidified with sulfuric acid to avoid cathodic separation of zinc and anodic deposition of thaUium(III) oxide [1314-32-5]. The metal deposited on the cathode is removed, kneaded into lumps, and dried. It is then compressed into blocks, melted under hydrogen, and cast into sticks. 

---