Arsenic elemental halogens

The great number of compounds of boron is due to the readiness with which boion atoms fonn, to some extent, chain structures with other boion atoms, and, to a far greater extent, cyclic compounds, both with other boron atoms, mid with atoms of carbon, oxygen, nitrogen, phosphorus, arsenic, the halogens, and many other elements. Examples of Lhcm arc shown below, beginning with the two pentaboranes B5 Hc> and Bs Hu ... 

For the non-metal compounds, the elements that are harmful to the catalyst are mostly main group A elements in the period table, that is, the VA group (N, P, As, Sb), the VIA group (O, S, Se, Te), the VIIA group (F, Cl, Br, I). Among these elements, phosphorus, arsenic, sulfur, halogens are the common poisons. 

A complete set of trihalides for arsenic, antimony and bismuth can be prepared by the direct combination of the elements although other methods of preparation can sometimes be used. The vigour of the direct combination reaction for a given metal decreases from fluorine to iodine (except in the case of bismuth which does not react readily with fluorine) and for a given halogen, from arsenic to bismuth. 

Elemental arsenic combines with many metals to form arsenides. When heated in the presence of halogens it forms trihaUdes however, pentahaUdes with the exception of AsF (2) and the unstable AsQ. are not readily formed. It reacts with sulfur to form the compounds AS2S2, AsS, As2S, and complex mixtures in various proportions (see Arsenic compounds). 

In a manner similar to phosphoms, arsenic, and antimony, the bismuth atom can be either tri- or pentacovalent. However, organobismuth compounds are less stable thermally than the corresponding phosphoms, arsenic, or antimony compounds, and there are fewer types of organobismuth compounds. For example, with R MX, R3MX2, R2MX3, and RMX, where M is a Group 15 (VA) element and X is a halogen, only the first two types have been prepared where M = Bi, but all four types are known where M = P, As, or Sb. 

The effect of a particular element on the odour of its compound seems also to lend support to the residual affinity theory, for it is only the elements which possess residual affinity in certain of their compounds, which function as osmophores. Oxygen, nitrogen, sulphur, phosphorous, halogens, arsenic, antimony, bismuth, etc., whose valencies vary under certain conditions are powerfully osmophoric whereas carbon, hydrogen, and many others which have a constant valency are practically non-osmophoric, and it is very instructive to note that the element is osmophoric when it is not employing its full number of valencies and therefore has free affinity. 

The magnetic criterion is particularly valuable because it provides a basis for differentiating sharply between essentially ionic and essentially electron-pair bonds Experimental data have as yet been obtained for only a few of the interesting compounds, but these indicate that oxides and fluorides of most metals are ionic. Electron-pair bonds are formed by most of the transition elements with sulfur, selenium, tellurium, phosphorus, arsenic and antimony, as in the sulfide minerals (pyrite, molybdenite, skutterudite, etc.). The halogens other than fluorine form electron-pair bonds with metals of the palladium and platinum groups and sometimes, but not always, with iron-group metals. 

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