Mendeleev's predicted elements

The four predicted elements lighter than the rare-earth elements, eka-boron (Eb, under boron, B, 5), eka-aluminium (Ea or El,[4] 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 (ekaborʺ), экаалюминій (ekaaljuminij), экамарганецъ (ekamarganecʺ), and экасилицій (ekasilicij) respectively, following the pre-1917 Russian orthography.

In 1871, Mendeleev predicted[4] the existence of a yet-undiscovered element he named eka-aluminium (because of its proximity to aluminium in the periodic table).

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'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.

[8] 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.

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.