Question

How could Mendeleev predict the properties of elements that had not yet been discovered

Answer 1

Mendeleev arranged the elements in accordance with he increasing of atomic mass. While he was categorizing then he found the gap in the elements in the atomic mass as according to the series the middle element's mass will be the average of the mass of other two elements and in some cases he found a large gap there. That's why he kept the places vacant and also confirmed that in later any new element could be put there which had yet not discovered.

Answer 2

Explanation:

in devising this table Mendeleev did not confirm completely to the order of atomic mass hi corrected the known atomic masses of some elements and he use the pattern in his table to predict the properties of the elements he thought must exist but had yet to be discovered