Atomic Weights of Silicon, History

Two distinct considerations generally have to be taken into account in determining the atomic weight of a solid element: (i) a decision has to be made as to the order of magnitude of the atomic weight; (ii) an exact estimation of the chemical equivalent of the element must be carried out by the employment of a suitable chemical reaction.

In the earlier days of atomic-weight determination the values obtained were often erroneous, both from ignorance of the physical laws which have to be taken into account, and from inadequate methods of chemical analysis. This applies particularly to the element silicon; and in tracing the history of the determination of the atomic weight of this element the development of our knowledge of physical methods that indicate the order of magnitude of its value will first be noticed, and then the chemical methods by which this value is determined with exactness.

Various formulae have been applied at different times to silica and its derivatives.

Thus Berzelius attempted to establish an analogy between potash alum and felspar, so that silica was written SiO₃ to agree with SO₃, the formula then given to sulphuric acid. Dumas in 1826 determined the vapour density of silicon chloride to be 5.94 (air = 1), which, when properly interpreted according to Avogadro's theory, would give a molecular weight of 171.7. Thence it might have been concluded that the chloride could not contain more than 4 atoms of chlorine. Nevertheless, this conclusion was overlooked, and the formula SiCl₆ was attributed to the chloride to agree with the formula SiO₃ for the oxide, to which the atomic weight Si = 44.4 corresponded. No fixed guiding principle had yet been established, however, and the formulae SiO and SiCl₂, with Si = 14.8, were adopted by Gmelin in 1826. In 1837 Kiihn advocated the formula SiO₂, which also found favour with mineralogists.

At different times during the course of about thirty years the formulae SiO, Si₂O₃, SiO₂, SiO₃ were in vogue, with the corresponding atomic weights of silicon of approximately 14, 21, 28, and 42: and this uncertainty existed not so much because adequate physical principles for atomic-weight determination had not been discovered as because these princip]es were misunderstood and perverted.

Gradually, however, a mass of evidence accumulated in favour of the formula SiO₂ and an atomic weight of 28.

H. Rose showed that the weight of silica which displaces CO₂ from alkali carbonates at high temperature corresponds with the formula SiO₂, and Marignac found that the isomorphism of fluosilicates with fluostannates supported the same formula, although the evidence of Dulong and Petit's law was inconclusive on account of the variation of specific heat with allotropy.

In 1859 Marignac extended the evidence in favour of the formula SiO₂ by showing the isomorphism of fluotitanates and fluozirconates with fluostannates and fluosilicates; and with the advent of the periodic law silicon found a place in the fourth group with carbon, titanium, zirconium, and tin. Thus an atomic weight of approximately 28 became definitely established for silicon.

The physical evidence for the atomic weight of silicon may thus be summarised:

1. Avogadro's Theory. - When the molecular proportion of any volatile silicon compound, as indicated by vapour-density determination, is analysed, 28 is the smallest proportion of silicon ever found within it.
2. Dulong and Petit's Law. - The atomic heat of silicon reaches the value 5.7 at 232° C. if the atomic weight is 28; and this is a normal value as compared with analogous elements.
3. The Law of Isomorphism. - In the complex fluorides of silicon, titanium, zirconium, and tin, 28 parts of silicon occupy the place of atomic proportions of the other elements.
4. The Periodic Law. - The atomic-weight value 28 enables silicon to be placed in the fourth group between aluminium and phosphorus, following carbon, and followed by titanium, zirconium, tin, lead, etc. This position is particularly suitable for an element exhibiting the properties possessed by silicon.

The first attempts to determine the atomic weight of silicon by chemical methods were made by Berzelius and Stromeyer, and were very imperfect.

The work of Pelouze in 1845 marked an advance on former methods. This chemist decomposed silicon tetrachloride with water, and estimated the chlorine by titration with silver nitrate. From his results the value 28.37 for the atomic weight of silicon is deducible, using modern antecedent data.

Dumas in 1859 adopted a similar method which yielded the value 28.08, whilst Schiel two years later employed the same reaction, but weighed the silver chloride. His results lead to the figure 27.95.

The above researches, however, are now merely of historical value. In 1887 Thorpe and Young hydrolysed carefully purified silicon tetra-bromide and weighed the silica obtained from it. They found the ratio

SiBr₄:SiO₂ = 100:17.347,

whence Si = 28.375.

Finally, in 1905, W. Becker and Julius Meyer treated very pure silicon tetrachloride in an analogous manner to the above, and obtained gelatinous silica therefrom, which was ignited and weighed as SiO₂. They found the ratio

SiCl₄:SiO₂ = 100:35.4145,

whence Si = 28.225.

So far as present knowledge goes there is nothing to choose between these two modern values. Hence the atomic weight of silicon is accepted by the International Atomic Weights Committee to be their mean, namely:

Si = 28.3.

It must be remembered, however, that the accumulated evidence for this figure is singularly slight, and that it cannot be regarded as certain even to the first decimal place.