Question

Why couldn't Mendeleev organize the entire periodic table during his research? Question options: The elements showed no periodic properties. The atomic number was unknown at the time. Some elements were undiscovered at the time. The atomic mass was unknown at the time.

Answer 1

Correct Answer:

Some elements were undiscovered at the time.

Explanation: This is straight from the test.

Answer 2

Pretty sure that some elements were discovered at the time is correct.