Question

multiple - choice question
what do the blank boxes represent?
elements mendeleev discovered
elements that are highly radioactive and could not be used safely at the time.
elements mendeleev said would be discovered later.

Explanation:

Mendeleev left blank boxes in his periodic - table for elements he predicted would be discovered in the future based on the periodic trends he observed. He did not discover the elements in the blank boxes himself, and the reason for the blanks was not related to radioactivity.

Answer:

C. elements Mendeleev said would be discovered later.