The four predicted elements lighter than the rare earth elements, ekaboron (Eb), ekaaluminium (El), ekamanganese (Em), and ekasilicon (Es), proved to be good predictors of the properties of scandium, gallium, technetium and germanium respectively, which each fill the spot in the periodic table assigned by Mendeleev. Initial versions of the periodic table did not give the rare earth elements the treatment now given them, helping to explain both why Mendeleev’s predictions for heavier unknown elements did not fare as well as those for the lightest predictions and why they are not as well known or documented.
Nowadays, the prefix eka- (and, more rarely, dvi-) is sometimes used in discussions about undiscovered elements, such as untriennium, also known as eka-actinium or dvi-lanthanum.
|melting point (°C)||Low||29.78|
|oxide's formula||Ea2O3 (density - 5.5 g cm-3) (soluble in both alkalis and acids)||Ga2O3 (density - 5.88 g cm-3) (soluble in both alkalis and acids)|
|chloride's formula||Ea2Cl6 (volatile)||Ga2Cl6 (volatile)|
|melting point (°C)||high||947|
|oxide type||refractory dioxide||refractory dioxide|
|oxide density (g/cm³)||4.7||4.7|
|oxide activity||feebly basic||feebly basic|
|chloride boiling point||under 100°C||86°C (GeCl4)|
|chloride density (g/cm³)||1.9||1.9|
Mendeleev's 1869 table had implicitly predicted a heavier analog of titanium and zirconium, but in 1871 he placed lanthanum in that spot. The 1923 discovery of hafnium validated Mendeleev's original 1869 prediction.
In 1902, having accepted the evidence for elements helium and argon, Mendeleev placed these Noble Gases in Group 0 in his arrangement of the elements. As Mendeleev was doubtful of atomic theory to explain the Law of definite proportions, he had no a priori reason to believe hydrogen 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 for radioactivity.
The heavier of the hypothetical proto-helium elements Mendeleev identified with coronium, named by association with an unexplained spectral line in the Sun's corona. A faulty calibration gave a wavelength of 531.68 nm, which was eventually corrected to 530.3 nm, which Grotrian and Edlen identified as originating from Fe XIV in 1939.
The lightest of the Zero Group gases, the first in the Periodic Table, was assigned a theoretical atomic mass between 5.3 x 10-11 and 9.6 x 10-7. 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 was so confident that these atomic elements would be discovered, that he included them in later publications of the periodic chart, although there was no physical evidence for their existence available at the time.
Mendeleev later published a theoretical expression of the ether, which satisfied many of the contradictions which existed in physics at that time, in a small booklet entitled, A Chemical Conception of the Ether, in 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 lighter-than-hydrogen elements. 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.
In 1905, Albert Einstein demonstrated that Brownian motion resulted as the natural consequence of the atomic theory. Later, atomic number was determined to be more natural than atomic weight in classifying the elements. While Mendeleev's genius persists in the arrangement of the periodic table, the lightest element was finally confirmed to be hydrogen, with atomic number 1.