There are only eight elements on earth which are present to the extent of 1 per cent or more, (viz. 0, Si, Al, Fe, Na, K, Mg, and Ca) and ofthese eight silicon is the only one which has properties of both a metal and a nonmetal. This circumstance Ieads to an unusually rich and diverse chemical behaviour, so that silicon spreads itself throughout ionic, covalent, organometallic, and colloidal chemistry. Our knowledge ofits mineral and ceramic chemistry stretches back ten thousand years, but the

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... ovalent chemistry of silicon has come a long way since Berzelius made silicon tetrachloride in 1823 and Friedeland Crafts made the first organosilicon compounds in 1863. The known covalent compounds now exceed 14,000 in number, and they are no Ionger confined to the laboratory, as Alfred Stock claimed, but have earned a place in polymer chemistry, in industry, in space exploration, and even in cosmetics. Tothose who know silicon, it is not at all surprising that organosilicon materials should prove their value in all these ways; the only point that is surprising is that it should all happen so quickly. It is only twenty-eight years since an active interest in organosilicon compounds began to take form, and the position as it now exists, is that to describe even the purely scientific side of the subject a three-volume compendium (Dr Bazant's) is required. In my talk this afternoon I shall touch upon a new and important aspect of organosilicon chemistry, the one that concerns the relation of silicon to nitrogen. Nitrogen is only 1/SOOOth as abundant as silicon, and even though there is so much of it in the atmosphere, it is not a natural partner in chemical combination with sil~con. Just as Alfred Stock's hydrides and Frederick Stanley Kipping's phenyl chlorosilanes were in their time, siliconnitrogen compounds are purely products of laboratory synthesis. This is a consequence of the greater bond formation energy of silicon-oxygen bonds, 89 kcalfmole, than silicon-nitrogen bonds, 77 kcalfmole, a circumstance which Ieads to the ready exchange of oxygen for nitrogen in most siliconnitrogen compounds. Some exceptions arise when many silicon atoms are bound to one nitrogen atom, notably in silicon nitride itself, a stable and unreactive but quite useless by-product of the commercial manufacture of silicon. Multiplebonding between silicon and nitrogen atoms is responsible for this effect, and although it is always a factor in the chemical behaviour of silicon-nitrogen compounds, it reaches overriding proportions only through the cumulative effect of several silicon atoms bound to one nitrogen atom.