Mendeleev's predicted elements
Dmitri Mendeleev published a periodic table of the 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
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 names were written by Dmitri Mendeleev as (), (), (), and () 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 ekaboron 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.
| 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 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.These are mass number of 97 and 98 which are different from an atomic mass in that they are counts of nucleons in the nuclei of some isotope and are not the atomic weight of an average sample (with a natural collection of isotopes). The 97Tc and 98Tc isotopes have respectively an atomic mass of 96.9063607 and 97.9072112, and respectively a half-life of years and years. For elements that are not stable enough to persist from the creation of the Earth, the convention is to report the atomic mass number of the most stable isotope in place of the naturally occurring atomic-mass average. .
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.
| 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 | |
Other predictions
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.
| 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.
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.
The heavier of the hypothetical proto-helium elements Mendeleev identified with coronium, named by association with an unexplained spectral line in the solar corona. A faulty calibration gave a wavelength of 531.68 nm, which was eventually corrected to 530.3 nm, which Walter Grotrian and Edlén identified as originating from iron XIV (i.e. Fe13+) in 1939.
The lightest of the Group 0 gases, the first in the periodic table, was assigned a theoretical atomic mass between and . 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.
Mendeleev later published a theoretical expression of the ether in a small booklet entitled A 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.
Later predictions
An English translation appeared as
Notes
Further reading
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