Dmitri Mendeleev published a periodic table of the chemical elements in 1869 based on properties that appeared with some regularity as he laid out the elements from lightest to heaviest. When Mendeleev proposed his periodic table, he noted gaps in the table and predicted that then-unknown elements existed with properties appropriate to fill those gaps. He named them eka-boron, eka-aluminium, eka-silicon, and eka-manganese, with respective atomic masses of 44, 68, 72, and 100.
Prefixes To give provisional names to his predicted elements, Dmitri Mendeleev used the prefixes eka- , dvi- or dwi-, and tri-, from the Sanskrit names of digits 1, 2, and 3, depending upon whether the predicted element was one, two, or three places down from the known element of the same group in his table. For example, germanium was called eka-silicon until its discovery in 1886, and rhenium was called dvi-manganese before its discovery in 1926. The eka- prefix was used by other theorists, and not only in Mendeleev's own predictions. Before the discovery, francium was referred to as eka-caesium, and astatine as eka-iodine. The official IUPAC practice is to use a systematic element name based on the atomic number of the element as the provisional name, instead of being based on its position in the periodic table as these prefixes required.
Original predictions The four predicted elements lighter than the rare-earth elements, eka-boron (Eb, under boron, B, 5), eka-aluminium (Ea or El, under Al, 13), eka-manganese (Em, under Mn, 25), and eka-silicon (Es, under Si, 14), proved to be good predictors of the properties of scandium (Sc, 21), gallium (Ga, 31), technetium (Tc, 43), and germanium (Ge, 32) respectively, each of which fill the spot in the periodic table assigned by Mendeleev. The names were written by Dmitri Mendeleev as экаборъ (ekabor), экаалюминій (ekaaljuminij), экамарганецъ (ekamarganec), and экасилицій (ekasilicij) respectively, following the pre-1917 Russian orthography. Initial versions of the periodic table did not distinguish rare earth elements from transition elements, helping to explain both why Mendeleev's predictions for heavier unknown elements did not fare as well as those for the lighter ones and why they are not as well known or documented. Scandium oxide was isolated in late 1879 by Lars Fredrick Nilson; Per Teodor Cleve recognized the correspondence and notified Mendeleev late in that year. Mendeleev had predicted an atomic mass of 44 for eka-boron in 1871, while scandium has an atomic mass of 44.955907. In 1871, Mendeleev predicted the existence of a yet-undiscovered element he named eka-aluminium (because of its proximity to aluminium in the periodic table). The table below compares the qualities of the element predicted by Mendeleev with actual characteristics of gallium, which was discovered, soon after Mendeleev predicted its existence, in 1875 by Paul Emile Lecoq de Boisbaudran.
Technetium was isolated by Carlo Perrier and Emilio Segrè in 1937, well after Mendeleev's lifetime, from samples of molybdenum that had been bombarded with deuterium nuclei in a cyclotron by Ernest Lawrence. Mendeleev had predicted an atomic mass of 100 for eka-manganese in 1871, and the most stable isotopes of technetium are 97Tc and 98Tc. Germanium was isolated in 1886 and provided the best confirmation of the theory up to that time, due to its contrasting more clearly with its neighboring elements than the two previously confirmed predictions of Mendeleev do with theirs.
Other predictions The existence of an element between thorium (90) and uranium (92) was predicted by Mendeleev in 1871. In 1900, William Crookes isolated a radioactive material deriving from uranium that he could not identify, which was later proven to be mixture of 234Th and 234mPa. Protactinium-234m (named "brevium") was identified in Germany in 1913, but the name protactinium was not given until 1918, when protactinium-231 was discovered. Since the acceptance of Glenn T. Seaborg's actinide concept in 1945, thorium, uranium and protactinium have been classified as actinides; hence, protactinium does not occupy the place of eka-tantalum (under 73) in group 5. Eka-tantalum is actually the synthetic superheavy element dubnium (105). Mendeleev's 1869 table had implicitly predicted a heavier analog of titanium (22) and zirconium (40), but in 1871 he placed lanthanum (57) in that spot. The 1923 discovery of hafnium (72) validated Mendeleev's original 1869 prediction.
Some other predictions were unsuccessful because he failed to recognise the presence of the lanthanides in the sixth row. In 1902, Bohuslav Brauner placed lanthanides in a special series instead of Mendeleev's extra period, so he renamed Mendeleev's tri-manganese as dvi-manganese and dvi-tellurium as eka-tellurium (polonium had already been discovered, but its chemical properties had not yet been studied). Dvi-caesium was renamed eka-caesium.
… excerpt ends here. Continue reading the full article.
