How was mendeleev able to predict the discovery of unknown elements?
Mendeleev arranged the elements in a table ordered by atomic
mass, corresponding to relative molar mass as defined today.
Mendeleev predicted the discovery of other elements and left space
for these new elements, namely eka-silicon (germanium),
eka-aluminium (gallium), and eka-boron (scandium). Thus, there was
no disturbance in the periodic table.