Chemical Properties of Silicon

Silicon forms with boron and carbon a little group of non-metals, which in the elementary state are characterised by distinct allotropy accompanied by high molecular complexity, so that not only are the elements non-volatile, even at high temperature, but they are, for the same reason, comparatively inert towards chemical reagents. Silicon in combination, like carbon, is almost invariably quadrivalent; though it appears that silica is reducible to a lower state of oxidation in which the element is bivalent. Silicon is one of the least electronegative of the non- metals, and forms compounds with other non-metals more readily than with metals; moreover, silicic acid is a very weak acid, and shares with boric, stannic, and some other acids the tendency to form condensed acids.

Silicon is the second member of the fourth group of the Periodic system; thus it occupies a position in the centre of the second short period. Like its neighbours in this period, it shows a relationship to the first member of the group - in this case carbon - which is by no means close, since all the members of the first short period show unique characteristics, which separate them from the rest of the elements. Especially is this the case with carbon, to which silicon, even with oar extending knowledge of its capabilities, will probably never be found to bear any close relationship. The relationship between silicon and other elements has been fully discussed in the introductory chapter.

The silicon present in a compound of this element is invariably detected and estimated in the form of silica, which is identified by the microcosmic or sodium carbonate bead test. In the former test the silica floats uncombined with the sodium metaphosphate in the clear bead of this substance formed by fusing microcosmie salt; in the latter test silica, by forming a glass, renders transparent the otherwise crystalline and opaque bead of sodium carbonate. The silica may be obtained, sometimes by hydrolysing the compound with water, by fusing it with a mixture of sodium and potassium carbonates and decomposing the resulting silicates with hydrochloric acid, or else by heating it with concentrated sulphuric acid. This latter method is useful for estimating the silicon in an organic compound; the acid burns up all the carbon and leaves a residue of pure silica.