Gallium prediction

Mann, J. B. to be published by the USSR Academic of Sciences, Institut of the History of Natural Sciences and Technology in their 1975 Centenary Volume of the Discovery of Gallium predicted by D. I. Mendeleev. 

In his opinion, a new previously unknown element contained in didymium was responsible for the appearance of the new lines in the spectrum. He named it decipium from the Latin to deceive, to stupefy and the name proved to be ironical decipium turned out to be a mixture of several REEs both known and unknown ones. Decipium was debunked in 1879 by L. de Boisbaudran of France who played a prominent role in the discovery of new REEs. In the next chapter we shall tell you how he discovered gallium predicted by Mendeleev. Boisbaudran extracted didymium from samarskite and thoroughly studied the sample by spectroscopy. Boisbaudran was a much more skillful experimenter than Delafontaine and he succeeded in separating the impurity from didymium . He named the new element samarium after samarskite, being unaware that samarium was also a mixture of elements. Boisbaudran s discovery was immediately confirmed by Marignac who, after multiple recrystallizations of samarium , separated two fractions which he marked Y and Yp (not to be confused with the symbol of yttrium Y ). The spectrum of the second fraction was identical to the spectrum of samarium . As to the first fraction, we shall have a look at it a little later. 

Gallium was predicted as eka-aluminium by D. 1. Mendeleev in 1870 and was discovered by P. E. Lecoq de Boisbaudran in 1875 by means of the spectroscope de Boi.sbaudran was, in fact, guided at the time by an independent theory of his own and had been searching for the missing element for some years. The first indications came with the observation of two new violet lines in the spark spectrum of a sample deposited on zinc, and within a month he had isolated 1 g of the metal starting from several hundred kilograms of crude zinc blende ore. The... 

Table 7.2 Comparison of predicted and observed properties of gallium...

The person whose name is most closely associated with the periodic table is Dmitri Mendeleev (1836-1907), a Russian chemist. In writing a textbook of general chemistry, Mendeleev devoted separate chapters to families of elements with similar properties, including the alkali metals, the alkaline earth metals, and the halogens. Reflecting on the properties of these and other elements, he proposed in 1869 a primitive version of today s periodic table. Mendeleev shrewdly left empty spaces in his table for new elements yet to be discovered. Indeed, he predicted detailed properties for three such elements (scandium, gallium, and germanium). By 1886 all of these elements had been discovered and found to have properties very similar to those he had predicted. 

And Maher goes on explicitly to underline the conclusions about confirmatory weight that he sees as illustrated by this episode. He claims that Mendeleev s prediction of the existence of the third of the new elements, eka-silicon (aka germanium), was initially regarded as quite unlikely to be true but then later, with the discovery of the first two new elements (gallium and scandium), confidence in the prediction of the existence of the third new element became so high that its eventual empirical confirmation was widely regarded as a matter of course. Maher writes ... 

Maher draws attention to the 1874 paper of Lecoq de Boisbaudran in which he announced the discovery of gallium, the first of Mendeleev s predictions to be confirmed. And this is, indeed, an important paper. One possible misconception should, however, be quashed immediately. De Boisbaudran did not discover gallium as a result of testing Mendeleev s prediction. Instead he operated quite independently by empirical means in ignorance of Mendeleev s prediction and he proceeded to characterise the new element spectroscopically. De Boisbaudran s findings were published in the Comptes Rendus. 

Lipton talks of the predictive successes with gallium and scandium as giving greater credence to Mendeleev s theory (a troublesome notion as already noted and as we shall see later in more detail) and only thence to the further prediction of germanium from that general theory. But Maher talks more directly of the impact of the successes with the first two new elements on the confidence that chemists... 

Mendeleev predicted that the melting point of gallium would fall between those of aluminium (660°C) and indium (115°C). In fact gallium has an anomalously low melting point of 30°C. 

After spending considerable time perusing the crumbling pages of late nineteenth-century chemistry journals and textbooks, I have confirmed the traditional account Mendeleev s periodic law attracted little attention. . . until chemists started to discover some of the elements needed to fill the gaps in his table and found that their properties were remarkably similar to those he had predicted. The frequency with which the periodic law was mentioned in-journals increased sharply after the discovery of gallium, most of that increase was clearly associated with Mendeleev s prediction of the properties of the new element, (p, 617)20... 

Firstly, although we have said it several times already, it may be as well to emphasise yet again that of course we agree that the predictive successes played an important role. As we indicated earlier, Mendeleev s prediction played no part in the actual discovery of gallium, the first new element. But in the case of scandium, the second new element, its discoverer, Cleve, remarked ... 

Mendeleev also predicted the existence of elements that had not yet been discovered. His arrangement of the then-known elements left some obvious holes in the periodic table. For instance, between zinc (combines with 2 Cl) and arsenic (combines with 5 Cl) were holes for one element that would combine with three chlorine atoms and another that would combine with four. Mendeleev assigned these holes to two new elements. He predicted that one element would have a molar mass of 68 g/mol and chemical properties like those of aluminum, while the other would have a molar mass of 72 g /mol and chemical properties similar to silicon. These elements, gallium (Z = 31, M M = 69.7 g/mol) and germanium (Z = 32, M M — 72.6 g/mol), were discovered within 15 years. Chemists soon verified that gallium resembles aluminum in its chemishy, while germanium resembles silicon, just as Mendeleev had predicted. 

The predictions made by Mendeleev provide an excellent example of how a scientific theory allows far-reaching predictions of as-yet-undiscovered phenomena. Today s chemists still use the periodic table as a predictive tool. For example, modem semiconductor materials such as gallium arsenide were developed in part by predicting that elements in the appropriate rows and columns of the periodic table should have the desired properties. At present, scientists seeking to develop new superconducting materials rely on the periodic table to identify elements that are most likely to confer superconductivity. 

C21-0043. From Its position in the periodic table, predict the properties of gallium (Element 31). 

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