Why couldn't mendeleev organize the entire periodic table durning his rea search

The answer is :

Some elements were undiscovered at this time.

The explanation:

- Mendeleev depended on the atomic mass of the element in his organization of the entire periodic table , and this made him to put all  the elements which have the same chemical and physical properties with each other into vertical column called groups

- and this method created a mean of a gaps in the rows, he assumed that those gaps represent the undiscovered elements ,but the new periodic table is organized by the increasing in atomic numbers.  

i believe the answer is true.