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Dmitri Mendeleev published aperiodic table of thechemical elements in 1869 based on properties that appeared with some regularity as he laid out the elements from lightest to heaviest.[1] 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.
To give provisional names to his predicted elements,Dmitri Mendeleev used the prefixeseka-/ˈiːkə-/,[note 1]dvi- ordwi-, andtri-, from theSanskrit names of digits 1, 2, and 3,[3] depending upon whether the predicted element was one, two, or three places down from the known element of the samegroup in his table. For example,germanium was called eka-silicon until its discovery in 1886, andrhenium was called dvi-manganese before its discovery in 1926.
Theeka- prefix was used by other theorists, and not only in Mendeleev's own predictions. Before the discovery,francium was referred to aseka-caesium, andastatine aseka-iodine. Sometimes, eka- is still used to refer to some of thetransuranic elements, for example,eka-radium forunbinilium. However, the current officialIUPAC practice is to use asystematic element name based on theatomic number of the element as the provisional name, instead of being based on its position in the periodic table as these prefixes require.
The four predicted elements lighter than therare-earth elements,eka-boron (Eb, under boron, B, 5),eka-aluminium (Ea orEl,[4] under Al, 13),eka-manganese (Em, under Mn, 25), andeka-silicon (Es, under Si, 14), proved to be good predictors of the properties ofscandium (Sc, 21),gallium (Ga, 31),technetium (Tc, 43), andgermanium (Ge, 32) respectively, each of which fill the spot in the periodic table assigned by Mendeleev.
The names were written by Dmitri Mendeleev asэкаборъ (ekabor),экаалюминій (ekaaljuminij),экамарганецъ (ekamarganec), andэкасилицій (ekasilicij) respectively, following thepre-1917 Russian orthography.
Initial versions of the periodic table did not distinguishrare earth elements fromtransition 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 byLars Fredrick Nilson;Per Teodor Cleve recognized the correspondence and notified Mendeleev late in that year. Mendeleev had predicted anatomic mass of 44 foreka-boron in 1871, while scandium has an atomic mass of 44.955907.
In 1871, Mendeleev predicted[4] the existence of a yet-undiscovered element he named eka-aluminium (because of its proximity toaluminium in theperiodic 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 byPaul Emile Lecoq de Boisbaudran.
| Property | Eka-aluminium | Gallium | |
|---|---|---|---|
| Atomic Mass | 68 | 69.723 | |
| Density (g/cm3) | 6.0 | 5.91 | |
| Melting point (°C) | Low | 29.76 | |
| Oxide | Formula | Ea2O3 | Ga2O3 |
| Density | 5.5 g/cm3 | 5.88 g/cm3 | |
| Solubility | Soluble in both alkalis and acids | ||
| Chloride | Formula | Ea2Cl6 | Ga2Cl6 |
| Volatility | Volatile | Volatile | |
Technetium was isolated byCarlo Perrier andEmilio Segrè in 1937, well after Mendeleev's lifetime, from samples ofmolybdenum that had been bombarded withdeuterium nuclei in acyclotron byErnest Lawrence. Mendeleev had predicted an atomic mass of 100 for eka-manganese in 1871, and the most stable isotopes of technetium are97Tc and98Tc.[5]
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.
| Property | Eka-silicon | Germanium | |
|---|---|---|---|
| Atomic Mass | 72 | 72.630 | |
| Density (g/cm3) | 5.5 | 5.323 | |
| Melting point (°C) | High | 938 | |
| Color | Grey | Grey | |
| Oxide | Type | Refractory dioxide | |
| Density (g/cm3) | 4.7 | 4.228 | |
| Activity | Feebly basic | Feebly basic | |
| Chloride | Boiling point | Under 100 °C | 86.5 °C (GeCl4) |
| Density (g/cm3) | 1.9 | 1.879 | |
The existence of an element betweenthorium (90) anduranium (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 of234Th and234mPa. Protactinium-234m (named "brevium") was identified in Germany in 1913,[6] but the nameprotactinium was not given until 1918, when protactinium-231 was discovered. Since the acceptance ofGlenn T. Seaborg'sactinide concept in 1945, thorium, uranium and protactinium have been classified asactinides; hence, protactinium does not occupy the place of eka-tantalum (under 73) ingroup 5. Eka-tantalum is actually thesyntheticsuperheavy elementdubnium (105).
Mendeleev's 1869 table had implicitly predicted a heavier analog oftitanium (22) andzirconium (40), but in 1871 he placedlanthanum (57) in that spot. The 1923 discovery ofhafnium (72) validated Mendeleev's original 1869 prediction.
| Mendeleev[7] | Modern names | Atomic Number |
|---|---|---|
| eka-boron | scandium, Sc | 21 |
| eka-aluminium | gallium, Ga | 31 |
| eka-silicon | germanium, Ge | 32 |
| eka-manganese | technetium, Tc | 43 |
| tri-manganese | rhenium, Re | 75 |
| dvi-tellurium | polonium, Po | 84 |
| dvi-caesium | francium, Fr | 87 |
| eka-tantalum | protactinium, Pa | 91 |
Some other predictions were unsuccessful because he failed to recognise the presence of the lanthanides in the sixth row.[7]
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.[8]
In 1902, having accepted the evidence for elementshelium andargon, Mendeleev placed these noble gases inGroup 0 in his arrangement of the elements.[9] As Mendeleev was doubtful ofatomic theory to explain thelaw of definite proportions, he had noa priori reason to believehydrogen was the lightest of elements, and suggested that a hypothetical lighter member of these chemically inert Group 0 elements could have gone undetected and be responsible forradioactivity. Currently some periodic tables of elements put loneneutrons in this place (seeneutronium) but no such element has ever been detected.
The heavier of the hypothetical proto-helium elements Mendeleev identified withcoronium, named by association with an unexplained spectral line in theSun's corona. A faulty calibration gave a wavelength of 531.68 nm, which was eventually corrected to 530.3 nm, whichGrotrian andEdlén identified as originating fromFe XIV (i.e. Fe13+) in 1939.[10][11]
The lightest of the Group 0 gases, the first in the periodic table, was assigned a theoretical atomic mass between5.3×10−11 Da and9.6×10−7 Da. The kinetic velocity of this gas was calculated by Mendeleev to be 2,500,000 meters per second. Nearly massless, these gases were assumed by Mendeleev to permeate all matter, rarely interacting chemically. The high mobility and very small mass of the trans-hydrogen gases would result in the situation that they could be rarefied, yet appear to be very dense.[12][13]
Mendeleev later published a theoretical expression ofthe ether in a small booklet entitledA Chemical Conception of the Ether (1904). His 1904 publication again contained two atomic elements smaller and lighter than hydrogen. He treated the "ether gas" as an interstellar atmosphere composed of at least two elements lighter than hydrogen. He stated that these gases originated due to violent bombardments internal to stars, the Sun being the most prolific source of such gases. According to Mendeleev's booklet, the interstellar atmosphere was probably composed of several additional elemental species.
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