Undiscovered elements

Ion exchange (qv see also Chromatography) is an important procedure for the separation and chemical identification of curium and higher elements. This technique is selective and rapid and has been the key to the discovery of the transcurium elements, in that the elution order and approximate peak position for the undiscovered elements were predicted with considerable confidence (9). Thus the first experimental observation of the chemical behavior of a new actinide element has often been its ion-exchange behavior—an observation coincident with its identification. Further exploration of the chemistry of the element often depended on the production of larger amounts by this method. Solvent extraction is another useful method for separating and purifying actinide elements. 

Of the remaining 26 undiscovered elements between hydrogen and uranium, 11 were lanthanoids which Mendeleev s system was unable to characterize because of their great chemical similarity and the new numerological feature dictated by the filling of the 4f orbitals. Only cerium, terbium and erbium were established with certainty in 1871, and the others (except promethium, 1945) were separated and identified in the period 1879 -1907. The isolation of the (unpredicted) noble gases also occurred at this time (1894-8). 

By explicitly showing the relationship between the elements, Mendeleyev was able to predict the existence and properties of elements that had not yet been discovered. He theorized, for example, that an undiscovered element should fall between silicon and tin on the periodic table. In 1880, German chemist Clemens Winkler isolated a new element, which he named germanium, that had exactly the properties that Mendeleyev predicted. 

The position on the periodic chart under lead would be filled by an element of atomic number 114. No such element is yet known, but scientists theorize that this would be a very stable element if it could be found or created, and it might have some very important uses. This much-sought element is referred to as i eka-lead, using the naming system Mendeleyev used for undiscovered elements in the original periodic chart. 

The discovery of these gases came as a great surprise to chemists. The periodic chart had predicted a few undiscovered elements, but no one expected a whole new group to be found. 

An odd property of many elements that helped scientists theoretically determine the characteristics of undiscovered elements is that elements having an even number of protons in their nuclei (atomic number) are more commonly found on Earth than are the elements with an odd number of protons. It is unclear why elements with odd numbers of protons in their nuclei are less commonly found than are those with even numbers of protons. 

It was Mendeleev who discovered the periodic law, a principle that describes the periodicities that are observed in the properties of the chemical elements. This enabled him to predict the existence of as-yet-undiscovered elements, to predict their atomic weights, and to describe their chemical and physical properties as well. It was Mendeleev who found the natural order in the elements that his predecessors Newlands, Chancourtis, and Odling had been seeking. Mendeleev couldn t explain why there were so many elements he didn t even try to do that. But he did discover the existence of striking patterns. 

Before continuing the story of the discovery of the chemical elements, it will be necessary to outline the early attempts at classification made by Dobereiner, Begtiyer de Chancourtois, and Newlands, and to discuss briefly the periodic system of the elements which teas developed independently by Lothar Meijer and Mendeleev. This classification enabled Mendeleev to predict the properties of a number of undiscovered elements and of their compounds with surprising accuracy, and proved to be of great assistance in all subsequent discoveries of new elements. 

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. 

One isotope of element 114, with 184 neutrons, is predicted to be another doubly magic nucleus, and is therefore expected to sit right in the middle of an island of stability in the space of superheavy nuclei (Fig. 13). Nuclear scientists suspect that it may have a half-life of as much as several years. Element 114 has thus become a kind of Holy Grail for element-makers. If it turns out to be stable, this would show that these researchers are not necessarily doomed to search for increasingly fleeting glimpses of ever heavier and less stable new elements. There might be undiscovered elements out there that you can (in principle, at least) hold in your hand. 

Let me try to rephrase the argument. We assume that the combination of a finite number of fundamental properties, via a combinatorial approach, leads to a discrete set of macroscopic physical possibilities. We also know empirically that the chemical elements occur in a discrete manner because there are no intermediate elements between, say, hydrogen and helium. The combinatorial approach can thus be taken as an explanation for the discreteness in the occurrence of elements and furthermore it justifies the fact that Mendeleev regarded the yet undiscovered elements like germanium as being physical possibilities rather than merely logical ones. 

A couple of years later, one of Becquerel s students, Marie Sklodowska Curie (1867—1934), shown in Figure 4.3, became keenly interested in this strange form of radiation. She showed that the radiation was also emitted by several other elements known at the time and suggested that it should be possible to isolate yet undiscovered elements by studying any radiation they might be emitting. Using chemical techniques, she and her husband, Pierre Curie... 

Thus, during the years after 1913, the feeling grew that the chemical properties of atoms could be pretty well understood. The idea that there were undiscovered elements, as indicated by gaps m the periodic system, was reinforced. These elements and more have since been discovered. 

Consider the as yet undiscovered elements with atomic numbers 115,117, and 119. 

What is the atomic number and expected ground-state electron configuration of the yet undiscovered element directly below Fr in the periodic table ... 

Figure 9.2 Mendeleev s periodic table. He left gaps for undiscovered elements.

The search for new elements must undoubtedly have been an appealing enterprise for young chemists around the turn of the century. Eventually, successful research could lead to the most praised reward a Nobel Prize. After the acceptance of the periodic law and system of Dimitrii Mendeleev, the search for yet undiscovered elements became a more organized and rationally based investigation, but nevertheless the identification and manufacture of new elements was made possible only by a set of techniques and instruments developed in the realm of both physics and chemistry. 

The first periodic table was developed in 1869 by Dmitri Mendeleev several decades before the nature of electron energy states in the atom was known. Mendeleev arranged the elements in order of increasing atomic mass into columns of similar physical and chemical properties. He then boldly predicted the existence and the properties of undiscovered elements to fill the gaps in his table. These interpolations were initially treated with skepticism until three of Mendeleev s theoretical elements were discovered and were found to have the properties he predicted. It is the correlation with properties—not with electron arrangements—that have placed the periodic table at the beginning of most chemistry texts. 

There are gaps in Mendeleev s table, some that he recognized as corresponding to hitherto undiscovered elements, and others to radioactive elements, many of which are artificial and short-lived. But his table represents one of the great advances in the understanding and systematization of chemistry, and it is an essential tool in teaching chemistry. 

The Periodic Table of The Chemical Elements (Table 2.3) was first organized by Mendeleyeff in 1869 [7] well before quantum mechanics and the modem theory of atomic structure, by using group analogies in chemical and physical properties Mendeleyeff even predicted two as yet undiscovered elements (Ga, Ge) and left spaces for them in his table. 

Here in Group III was a gap between calcium and titanium. Since it occurred under boron, the missing element must resemble boron. This was his eka-boron which he predicted. There was another gap in the same group under aluminum. This element must resemble aluminum, so he called it eka-aluminum. And finally he found another vacant space between arsenic and eka-aluminum, which appeared in the fourth group. Since its position was below the element silicon, he called it eka-silicon. Thus he predicted three undiscovered elements and left it to his chemical contemporaries to verify his prophecies. Not such remarkable guesses after all—at least not to the genius Mendel eff ... 

I shall endeavor to show, as briefly as possible, in how far the periodic law contributes to enlarge our range of vision. Before the promulgation of this law the chemical elements were mere fragmentary, incidental facts in Nature there was no special reason to expect the discovery of new elements, and the new ones which were discovered from time to time appeared to be possessed of quite novel properties. The law of periodicity first enabled us to perceive undiscovered elements at a distance which formerly was inaccessible to chemical vision... 

That each element fits properly into place in a vertical column proves the fundamental correctness of arranging the elements according to their atomic numbers and chemical properties. Henry Moseley (1887-1915) discovered a quantitative relationship between the wavelength of X-rays emitted by an element and the atomic number of the element. Every atomic number between 1 and 92 was accounted for, which means that there are no more undiscovered elements except possibly artificial elements with very high atomic nnmbers yet to be synthesized. 

Although Mendeleev predicted the existence of several undiscovered elements, he did not predict the existence of the noble gases, the lanthanides, or the actinides. Propose reasons why Mendeleev was not able to predict the existence of the noble gases. 

---