Throughout history, scientists have been known to copy ideas from their peers. Dmitry Mendeleev was no exception—he arranged all of the elements known at the time according to their atomic masses, just as Johann Döbereiner and John Newlands did before him. However, Mendeleev intentionally left blank spaces in his working version of the periodic table of elements.

Which statement best explains the importance of the blank spaces in Mendeleev’s version of the periodic table of elements?