Question

On what conditions or by comparing what conditions does Mendeleev was able to predict the properties for the undiscovered element and left the gap for them

Answer 1

Mendeleev found that the properties of elements were related to atomic mass in a periodic way. He arranged the elements such that elements with similar properties fell into same vertical columns of the periodic table.

He formulated a periodic law which states:

“the properties of elements are the periodic function of their atomic masses.”
Based on the same periodic law he left some gaps for the elements yet to be discovered. So that, if a certain new element is discovered, it can be placed in a new group without disturbing any existing group.
He recognized using the trends in his periodic table to predict the existence and properties of unknown elements such as aluminium, boron, silicon which he called eka-aluminum, eka-boron, and eka-silicon. The elements gallium, scandium and germanium which were found later also fit into the left gaps.