When Mendeleev proposed his Periodic Table, he noted gaps in the table, and ... Initial versions of the periodic table did not give the rare earth elements the ... that these atomic elements would be discovered, that he included them in later.
Those gaps were later filled by elements discovered after the creation of the table, which justified the gaps being left for that purpose.
The gaps were left for elements which were not discovered at that time. The properties which were estimated for the elements at that time was same as found later in those elements.
The gaps estimated their chemical properties.The elements when discovered were found to have those properties.They were then placed in these gaps.
Mendeleev left those gaps for the elements which were not discovered at that time. He named those elements like eka-aluminium,eka -silicon etc.
He predicted that elements with the predicted properties would be discovered to fill in these gaps.
The gaps in the first periodic elements described those elements which were not discovered at that time. Eg-silicon,germanium etc
Mendeleev left some gaps in his table for the elements which were not discovered at that time.Those elements were later found as silicon,gallium,germanium which were then filled in the gaps
The gaps represented the elements which were not discovered. When the elements were invented, they were placed into these gaps.
Because at that time there were no known elements to fill those gaps. The fact that they were filled indicated the power of his classification method. When a scientific theory predicts things clearly and these are later found, the theory becomes validated.
Mendeleev left gaps for undiscovered elements. He predicted their properties which were found to be accurate later.
Scientists suggest Mendeleev left gaps in the periodic table because Mendeleev knew the elements existed; however, was unable to prove it.
He made the periodic table. When he found gaps he said there were unknown elements, he was right and those elements were found in his lifetime.Mendeleev was the first person to organize the elements into what we would recognize as a version of the modern periodic table.