Some elements predicted by Mendeleev

Three of the undiscovered elements whose properties Mendeleev foretold in great detail, ekaaluminum, ekaboron, and ekasilicon, were discovered within fifteen years from the time of their prediction. The first was found by Lecoq de Boisbaudran m France, the second by Lars Frednk Nilson in Sweden, and the third by Clemens Winkler in Germany. These elements were named gallium, scandium, and germanium in honor of these countries. 

VV hen Mendeleev predicted that occupants would be found for the vacant spaces in the periodic system, he little dreamed that three of them would be discovered during his lifetime. 

The finding of this element was by no means accidental. Boisbaudran had been studying spectra for fifteen years, and had found that in 

French chemist who discovered gallium, samarium, and dysprosium, and perfected methods of separating the rare earths He ranks with Bunsen, Kirch-hofiF, and Crookes as one of the founders of the science of spectroscopy. 

The following account of the discovery was written by Boisbaudran himself for Chemical News  

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Francium was discovered in 1939, but its existence was predicted by Mendeleev in the 1870s. It is the least stable of the first 101 elements Its most stable isotope has a half-life of just 22 minutesi Use your knowledge about the properties of other alkali metals to predict some of francium s properties. 

The time of discovery of gallium is known to an hour. On Friday of August 27, 1875, between 3 p.m. and 4 p.m. I discovered some signs that there can be a new simple body in the by-product of chemical analysis of zinc blende from the Pierfitt mine in the Argele valley (Pyrenees). With these words P. E. Lecoq de Boisbaudran began his report to the Paris Academy of Sciences. He described some of the new element s properties and noted that its presence in the ore was ascertained by spectral analysis just as predicted by Mendeleev five years before. Boisbaudran extracted an extremely small amount of the substance and, therefore, could not study its properties properly. 

Some elements were unknown at the time of Mendeleev. These elements included all of the inert gases, most of the inner transition elements, and others scattered about the table, such as scandium, gallium, and germanium. Mendeleev had sufficient confidence in the periodicity principle that he left blank spaces in his table for undiscovered elements. These blanks were later filled in with elements having much the same properties as those predicted by Mendeleev. 

The concept of chemical periodicity is central to the study of inorganic chemistry. No other generalization rivals the periodic table of the elements in its ability to systematize and rationalize known chemical facts or to predict new ones and suggest fruitful areas for further study. Chemical periodicity and the periodic table now find their natural interpretation in the detailed electronic structure of the atom indeed, they played a major role at the turn of the century in elucidating the mysterious phenomena of radioactivity and the quantum effects which led ultimately to Bohr s theory of the hydrogen atom. Because of this central position it is perhaps not surprising that innumerable articles and books have been written on the subject since the seminal papers by Mendeleev in 1869, and some 700 forms of the periodic table (classified into 146 different types or subtypes) have been proposed. A brief historical survey of these developments is summarized in the Panel opposite. 

There are a number of anomalies in the long form, whose exposition has caused the spilling of much ink. One of Mendeleev s greatest triumphs was his prediction of a new element ( eka-silicon ) between Si and Sn in his Table. Mendeleev had the audacity to predict some chemical properties of this new element, and his prophecies were substantially fulfilled a few years later by the isolation of germanium and a preliminary exploration of its chemistry. These predictions were made simply by interpolation between Si and Sn. Chemists, and chemistry students, have come to expect that the chemical properties within a Group follow monotonic trends properties can be predicted by interpolation and extrapolation. Experimental observations which do not fit such simple trends lead to the identification of anomalies . At one extreme, there may be a tendency to sweep such anomalies discreetly under the carpet, or even to question the validity of the data at the other extreme, strenuous efforts are made to account for anomalies by means of elaborate and sometimes fanciful theorising. 

Why Mendeleev s table became the norm is a question for serious historical scholarship, but some pieces of the story are clear. The table was a convenient and efficient way to facilitate certain pattern-based forms of chemical reasoning. This power was most forcefully displayed by Mendeleev s bold predictions of new elements to fill in the gaps of his table. But the role of the table in modern chemical practice, while perhaps less spectacular, is no less compelling. How it works should command both philosophical and psychological attention. 

When Mendeleev arranged the elements according to their atomic masses, some elements didn t fit. He resolved this problem by predicting the existence and properties of elements that were unknown at the time. In this MinrLab, you ll predict some properties of two unknown elements. 

Since many elements had not yet been discovered, Mendeleev s table contained some gaps, which allowed him to predict the existence (and even the properties) of yet undiscovered elements. For example, Mendeleev predicted the existence of an elanent he called eka-silicon, which fell below silicon on the table and between gallium and arsenic. In 1886, eka-silicon was discovered by German chemist Clemens Winkler (1838-1904), who named it germanium, after his home country. 

The priority dispute took place in the Berichte derDeutschen Chemischen Gesellschaft and could be easily read by the chemical community, which at the time was dominated by organic chemists. How did the chemical community respond to the priority dispute At first, no other papers could be found that explicitly discussed the priority dispute. It cannot be excluded that the correspondences reflected the dispute." But it is obvious that the authors who dealt directly or indirectly with the questions of atomism, elements, and their classification or the determination of atomic weights often mentioned the successful predictions of Mendeleev and showed some reticence concerning Meyer s work. It seems that the success of (some) predictions of new elements influenced the process of forgetting about Meyer s precautionary contribution and overemphasizing the role of the predictions in the process of acceptance. ... 

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