Question

Due to uncertainty in the names of some of elements, a question mark is indicated before the symbol in the Mendeleev's periodic table. What are such symbols?

Answer 1

The most probable reason is that because when Mendeleev started to sort the elements into what would become the periodic table, not all the elements had been discovered.

He recognized that based on periodicity, certain elements are likely to exist, they just hadn't been discovered yet.

Hence those symbols came with a question mark.

Answer 2

Answer:

Memdeleevv indicated these elements as que mark and other signs