Prediction of Unknown Chemical Elements
The history of gallium, scandium and germanium shows that their discoveries were practically unaffected by the periodic law and periodic system. However, the properties predicted by D.I Mendeleev for eka-aluminium, eka-boron and eka-silicon coincided with those of gallium, scandium, and germanium. Mendeleev had determined the main features of these elements long before they were discovered in nature. Is not this fact a striking evidence of the periodic system’s power of prediction?
The discovery of gallium and its identity with eka-aluminium became milestones in the history of the periodic law and in the history of discovery of elements. After 1875 even those scientists who had disregarded the periodic system had to recognize its value. And among them there were top researchers, such as R. Bunsen, the creator of spectral analysis (he once said that to classify elements is the same thing as to search for regularities in the stock-exchange quotations) or P. Cleve who had never mentioned the periodic system in his lectures. The discovery of scandium and germanium meant further triumph of Mendeleev’s theory of periodicity.
In addition to the classic triad Mendeleev predicted the existence other unknown elements. On the whole, as early as 1870 Mendeleev saw about ten vacant places in his table. He saw them, for instance, in the seventh group where there were neither manganese analogues nor a heavy iodine’s analogue (the heaviest halogen which had to possess metallic properties).
In Mendeleev’s papers we find mention of eka-, dvu-, and tri-manganese and eka-iodine. The scientist firmly believed in their existence. And here we encounter a very interesting fact in the predictions. Eka-manganese (known subsequently as technetium) and eka–iodine (astatine) were synthesized later. Mendeleev, naturally, could not know that they did not exist in nature and firmly believed in their existence since these elements filled in the gaps in the periodic system and made it more logical.
The prediction consists of two stages: prediction of the existence of an element and prediction of its main properties. The first stage was in many respects guess-work for Mendeleev. As yet unknown was the phenomenon of radioactivity making some elements so short-lived that their earthly existence is impossible at all or they exist only because they are products of radioactive transformations of long-lived elements (thorium and uranium).
The second stage was completely within Mendeleev’s power and depended on his confidence. Sometimes Mendeleev predicted boldly and resolutely. This was the case with eka-aluminum, eka-boron, and eka-silicon: these elements had to be placed in that part of the periodic table where many well-known and well-studied elements had already been located–the region of reliable prediction. Sometimes Mendeleev predicted the properties of unknown elements with the extreme caution. Among them were analogues of manganese, iodine, and tellurium as well as the missing elements of the beginning of the seventh period: eka-cesium, eka-barium, eka-lanthanum, and eka-tantalum. Here Mendeleev was groping in the dark, darling only to estimate atomic masses and suggest formulas of oxides. Mendeleev thought that it was difficult to predict the properties of the unknown elements (including those of REEs) whose places were at the boundaries of the system because there were few known elements around them. This was the “grey” area of uncertain prediction. Of course, they included the rare–earth elements. Finally, in some parts of the periodic table prediction was completely unreliable. They included those mysterious stretches extending in the directions of hypothetical elements lighter than hydrogen and heavier than uranium. Mendeleev never thought that the periodic system had to begin with hydrogen. He even wrote a paper in which he described two elements preceding hydrogen. Only when physicists explained the meaning of the periodic law, his mistake became clear: the nucleus of the hydrogen atom had the smallest charge equal to 1. As regards elements which are heavier than uranium, Mendeleev conceded the existence of a very restricted number of them and never took the liberty of predicting, even approximately, their possible properties. Predictions of this kind did not come until much later when they signaled important events in the history of science.