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.