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CHAPTER XVIII

SILICON AND THE OTHER ELEMENTS OF THE FOURTH GROUP

CARBON, which gives the compounds CH, and CO2, belongs to the fourth group of elements. The nearest element to carbon is silicon, which forms the compounds SiH, and SiO2; its relation to carbon is like that of aluminium to boron or phosphorus to nitrogen. As carbon composes the principal and most essential part of animal and vegetable substances, so is silicon almost an invariable component part of the rocky formations of the earth's crust. Silicon hydride, SiH4, like CH ̧, has no acid properties, but silica, SiO2, shows feeble acid properties like carbonic anhydride. In a free state silicon is also a non-volatile, slightly energetic non-metal, like carbon. Therefore the form and nature of the compounds of carbon and silicon are very similar. In addition to this resemblance, silicon presents one exceedingly important distinction from carbon namely, the nature of the higher degree of oxidation. That is, silica, silicon dioxide, or silicic anhydride, SiO2 is a solid, non-volatile, and exceedingly infusible substance, very unlike carbonic anhydride, CO2, which is a gas. This expresses the essential peculiarity of silicon. The cause of this distinction may be most probably sought for in the polymeric composition of silica compared with carbonic anhydride. The molecule of carbonic anhydride con tains CO2, as seen by the density of this gas. The molecular weight and vapour density of silica, were it volatile, would probably correspond with the formula SiO2, but it might be imagined that it would correspond to a far higher atomic weight of SiO2, principally from the fact that SiH, is a gas like CH4, and SiCl, is a liquid and volatile, boiling at 57°—that is, even lower than CC14, which boils at 76°. In general, analogous compounds of silicon and carbon have nearly the same boiling points if they are liquid and volatile. From this it might

1 Chloroform, CHCl3, boils at 60°, and silicon chloroform, SiHCl3, at 84°; silicon ethyl, Si(C2H5)4, boils at about 150°, and its corresponding carbon compound, C(CH3)4) at about 120°; ethyl orthosilicate, Si(OC2H5)4, boils at 160°, and ethyl orthocarbonate, C(OC2H5)4, at 158°. The specific volumes in a liquid state-that is, those of the silicon compounds generally are slightly greater than those of the carbon compounds; for

be expected that silicic anhydride, SiO2, would be a gas like carbonic anhydride, whilst in reality silica is a hard non-volatile substance, lbis and therefore it may with great certainty be considered that in this condition it is polymeric with SiO2, as on polymerisation-for instance, when cyanogen passes into paracyanogen, or hydrocyanic acid into cyanuric acid (Chapter IX.)—very frequently gaseous or volatile substances change into solid, non-volatile, and physically denser and more complex substances. We will first make acquaintance with free silicon and its volatile compounds, as substances in which the analogy of silicon with carbon is shown, not only in a chemical but also in a physical sense.3

example, the volumes of CCl4-94, SiCl=112, CHCl3=81, SiHCl=82, of C(OCH25)4 =186, and Si(OC2H5)=201. The corresponding salts have also nearly equal specific volumes; for example, CaCO3=37, CaSiO3=41. It is impossible to compare SiO2 and CO2, because their physical states are so widely different.

1 bis But silica fuses and volatilises (Moissan) in the heat of the electric furnace, about 3000°, SiO2 is also partially volatile at the temperature attained in the flame of detonating gas (Cremer, 1892).

2 A property of intercombination is observable in the atoms of carbon, and a faculty for intercombination, or polymerisation, is also seen in the unsaturated hydrocarbons and carbon compounds in general. In silicon a property of the same nature is found to be particularly developed in silica, SiO2, which is not the case with carbonic anhydride. The faculty of the molecules of silica for combining both with other molecules and among themselves is exhibited in the formation of most varied compounds with bases, in the formation of hydrates with a gradually decreasing proportion of water down to anhydrous silica, in the colloid nature of the hydrate (the molecules of colloids are always complex), in the formation of polymeric ethereal salts, and in many other properties which will be considered in the sequel. Having come to this conclusion as to the polymeric state of silica since the years 1850-1860, I have found it to be confirmed by all subsequent researches on the compounds of silica, and, if I mistake not, this view has now been very generally accepted.

3 It was only after Gerhardt, and in general subsequently to the establishment of the true atomic weights of the elements (Chapter VII.), that a true idea of the atomic weight of silicon and of the composition of silica was arrived at from the fact that the molecules of SiCl4, SiF4, Si(OC2H5), &c., never contain less than 28 parts of silicon.

The question of the composition of silica was long the subject of the most contradictory statements in the history of science. In the last century Pott, Bergmann, and Scheele distinguished silica from alumina and lime. In the beginning of the present century Smithson for the first time expressed the opinion that silica was an acid, and the minerals of rocks salts of this acid. Berzelius determined the presence of oxygen in silica-namely, that 8 parts of oxygen were united with 7 of silicon. The composition of silica was first expressed as SiO (and for the sake of shortness S only was sometimes written instead). An investigation in the amount of silica present in crystalline minerals showed that the amount of oxygen in the bases bears a very varied proportion to the amount of oxygen in the silica, and that this ratio varies from 2:1 to 1:3. The ratio 1 1 is also met with, but the majority of these minerals are rare. Other more common minerals contain a larger proportion of silica, the ratio between the oxygen of the bases and the oxygen of the silica being equal to 1: 2, or thereabouts; such are the augites, labradorites, oligoclase, talc, &c. The higher ratio 1: 3 is known for a widely distributed series of natural silicates-for example, the felspars. Those silicates in which the amount of oxygen in the bases is equal to that in the silica are termed monosilicates ;

=

Free silicon can be obtained in an amorphous or crystalline state. Amorphous silicon is produced, like aluminium, by decomposing the double fluoride of sodium and silicon (sodium silicofluoride) by means of sodium: Na,SiF+4Na 6NaF+Si. By treating the mass thus obtained with water the sodium fluoride may be extracted and the residue will consist of brown, powdery silicon. In order to free it from any silica which might be formed, it is treated with hydrofluoric acid. This silicon powder is not lustrous; when heated it easily ignites, but does not completely burn. It fuses when very strongly heated, and

their general formula will be (RO),SiO2 or (R2O3)2(SiO2)3. Those in which the ratio of the oxygen is equal to 1:2 are termed bisilicates, and their general formula will be ROSIO2 or R03 (SiO2)3. Those in which the ratio is 1 : 3 will be trisilicates, and their general formula (RO)2(SiO2)3 or (R9O3)2(SiO2)9.

In these formula the now established composition of SiO2-that is, that in which the atom of Si = 28-is employed. Berzelius, who made an accurate analysis of the composition of felspar, and recognised it as a trisilicate formed by the union of potassium oxide and alumina with silica, in just the same manner as the alums are formed by sulphuric acid, gave silica the same formula as sulphuric anhydride—that is, SiO3. In this case the formula of felspar would be exactly similar to that of the alums-that is, KAl(SiO4)2, like the alums, KAI(SO4)2. If the composition of silica be represented as SiO3, the atom of silicon must be recognised as equal to 42 (if O=16; or if O=8, as it was before taken to be, Si = 21).

The former formulæ of silica, SiO(Si = 14,) and SiO3(Si=42), were first changed into the present one, SiO(Si = 28), on the basis of the following arguments:-An excess of silica occurs in nature, and in siliceous rocks free silica is generally found side by side with the silicates, and one is therefore led to the conclusion that it has formed acid salts. It would therefore be incorrect to consider the trisilicates as normal salts of silica, for they contain the largest proportion of silica; it is much better to admit another formula with a smaller proportion of oxygen for silica, and it then appears that the majority of minerals are normal or slightly basic salts, whilst some of the minerals predominating in nature contain an excess of silica--that is, belong to the order of acid salts.

At the present time, when there is a general method (Chapter VII.) for the determination of atomic weights, the volumes of the volatile compounds of silica show that its atomic weight Si=28, and therefore silica is SiO2. Thus, for example, the vapour density of silicon chloride with respect to air is, as Dumas showed (1862), 5'94, and hence with respect to hydrogen it is 85'5, and consequently its molecular weight will be 171 (instead of 170 as indicated by theory). This weight contains 28 parts of silicon and 142 parts of chlorine, and as an atom of the latter is equal to 35'5, the molecule of silicon chloride contains SiCl4. As two atoms of chlorine are equivalent to one of oxygen, the composition of silica will be SiO2-that is, the same as stannic oxide, SnO2, or titanic oxide, TiO2, and the like, and also as carbonic and sulphurous anhydrides, CO2 and SO2. But silica bears but little physical resemblance to the latter compounds, whilst stannic and titannic oxides resemble silica both physically and chemically. They are non-volatile, crystalline, insoluble, are colloids, also form feeble acids like silica, &c., and they might therefore be expected to form analogous compounds, and be isomorphous with silica, as Marignac (1859) found actually to be the case. He obtained stannofluorides, for example an easily soluble strontium salt, SrSnF6, 2H2O, corresponding with the already long known silicofluorides, such as SrSiF, 2H2O. These two salts are almost identical in crystalline form (monoclinic; angle of the prism, 83° for the former and 84° for the latter; inclination of the axes, 103° 46′ for the latter and 103° 30' for the former), that is, they are isomorphous. We may here add that the specific volume of silica in a solid form is 22:6, and of stannic oxide 21.5.

has then the appearance of carbon. Crystalline silicon is obtained in a similar way, but by substituting an excess of aluminium for the sodium 3Na,SiF+4A1 = 6NaF+4A1F3+3Si. The part of the aluminium remaining in the metallic state dissolves the silicon, and the latter separates from the solution on cooling in a crystalline form. The excess of aluminium after the fusion is removed by means of hydrochloric and hydrofluoric acid. The best silicon crystals are obtained from molten zinc; 15 parts of sodium silicofluoride are mixed with 20 parts of zinc and 4 parts of sodium, and the mixture is thrown into a strongly heated crucible, a layer of common salt being used to cover it; when the mass fuses it is stirred, cooled, treated with hydrochloric acid, and then washed with nitric acid. Silicon, especially when crystalline, like graphite and charcoal, does not in any way act on the above-mentioned acids. It forms black, very brilliant, regular octahedra having a specific gravity of 2-49; it is a bad conductor of electricity, and does not burn even in pure oxygen (but it burns in gaseous fluorine). The only acid which acts on it is a mixture of hydrofluoric and nitric acids; but caustic alkalis dissolve in it like aluminium, with evolution of hydrogen, thus showing its acid character. In general silicon strongly resists the action of reagents, as do also boron and carbon. Crystalline silicon was obtained in 1855 by Deville, and amorphous silicon in 1826 by Berzelius. 4 bis

Silicon hydride, SiH, analogous to marsh gas, was obtained first of all in an impure state, mixed with hydrogen, by two methods: by the action of an alloy of silicon and magnesium on hydrochloric acid," and by the action of the galvanic current on dilute sulphuric acid, using electrodes of aluminium, containing silicon. In these cases

4 A similar form of silicon is obtained by fusing SiO, with magnesium, when an alloy of Si and Mg is also formed (Gattermann). Warren (1888) by heating magnesium in a stream of SiF, obtained silicon and its alloy with magnesium. Winkler (1890) found that Mg,Si, and Mg,Si are formed when SiO2 and Mg are heated together at lower temperatures, whilst at a high temperature Si only is formed.

4bis It is very remarkable that silicon decomposes carbonic anhydride at a white heat, forming a white mass which, after being treated with potassium hydroxide and hydrofluoric acid, leaves a very stable yellow substance of the formula SiCO, which is formed according to the equation, 3Si+2CO2 = SiO2+2SICO. It is also slowly formed when silicon is heated with carbonic oxide. It is not oxidised when heated in oxygen. A mixture of silicon and carbon when heated in nitrogen gives the compound Si,C,N, which is also very stable. On this basis Schützenberger recognises a group, C.Si, as capable of combining with O and N, like C.

We may add that Troost and Hautefeuille, by heating amorphous silicon in the vapour of SiCl4, obtained crystalline silicon, and probably at the same time lower compounds of Si and Cl were temporarily formed. In the vapour of TiCl, under the same conditions crystalline titanium is formed (Levy, 1892).

5 This alloy, as Beketoff and Cherikoff showed, is easily obtained by directly heating finely divided silica (the experiment may be conducted in a test tube) with magnesium

silicon hydride is set free, together with hydrogen, and the presence of the hydride is shown by the fact that the hydrogen separated ignites spontaneously on coming into contact with the air, forming water and silica. The formation of silicon hydride by the action of hydrochloric acid on magnesium silicide is perfectly akin to the formation of phosphuretted hydrogen by the action of hydrochloric acid on calcium phosphide, to the formation of hydrogen sulphide by the action of acids on many metallic sulphides, and to the formation of hydrocarbons by the action of hydrochloric acid on white cast iron. On heating silicon hydride-that is, on passing it through an incandescent tube, it is decomposed into silicon and hydrogen, just like the hydrocarbons, but the caustic alkalis, although without action on the latter, react with silicon hydride according to the equation : SiH,+2KHO+H2O=SiK2O3+4H2.

Silicon chloride, SiCl4, is obtained from amorphous anhydrous silica (made by igniting the hydrate) mixed with charcoal, heated to a white

powder (Chapter XIV., Notes 17, 18). The substance formed, when thrown into a solution of hydrochloric acid, evolves spontaneously inflammable and impure silicon hydride, so that the self-inflammability of the gas is easily demonstrated by this means.

In 1850-60 Wöhler and Buff obtained an alloy of silicon and magnesium by the action of sodium on a molten mixture of magnesium chloride, sodium silicofluoride, and sodium chloride. The sodium then simultaneously reduces the silicon and magnesium.

Friedel and Ladenburg subsequently prepared silicon hydride in a pure state, and showed that it is not spontaneously inflammable in air, at the ordinary pressure, but that, like PH3, and like the mixture prepared by the above methods, it easily takes fire in air under a lower pressure or when mixed with hydrogen. They prepared the pure compound in the following manner: Wöhler showed that when dry hydrochloric acid gas is passed through a slightly heated tube containing silicon it forms a very volatile colourless liquid, which fumes strongly in air; this is a mixture of silicon chloride, SiCl4, and silicon chloroform, SiHCl3, which corresponds with ordinary chloroform, CHC15. This mixture is easily separated by distillation, because silicon chloride boils at 57°, and silicon chloroform at 36°. The formation of the latter will be understood from the equation Si+3HCl = H2+ SiHCl. It is an anhydrous inflammable liquid of specific gravity 16. It forms a transition product between SiH4 and SiCl4, and may be obtained from silicon hydride by the action of chlorine and SbCl, and is itself also transformed into silicon chloride by the action of chlorine. Gattermann obtained SiHCl, by heating the mass obtained after the action (Note 4) of Mg upon SiO2, in a stream of chlorine (with HCl) at about 470°. Friedel and Ladenburg, by acting on anhydrous alcohol with silicon chloroform, obtained an ethereal compound having the composition SiH(OCH5)5. This ether boils at 136°, and when acted on by sodium disengages silicon hydride, and is converted into ethyl orthosilicate, Si(OC2H5)4, according to the equation 4SiH(OC2H5)3 - SiH,+3Si(OC1H5), (the sodium seems to be unchanged), which is exactly similar to the decomposition of the lower oxides of phosphorus, with the evolution of phosphuretted hydrogen. If we designate the group C2H5, contained in the silicon ethers by Et, the parallel is found to be exact:

4PHO(OH), PH3+3PO(OH)3; 4SiH(OEt); = SiH4 +3Si(OEt).

6 The amorphous silica is mixed with starch, dried, and then charred by heating the mixture in a closed crucible. A very intimate mixture of silica and charcoal is thus