Mendeleev left gaps in his table of the elements. What did he believe those gaps meant?

1. Mendeleev arranged the elements he knew by mass and by their properties. 2. A repeating pattern appeared in those properties as he went along. 3. Some places in the pattern had no known element to fill them. 4. Rather than change the pattern, he left those places empty. 5. He argued the gaps stood for elements nobody had found yet. 6. Later discoveries filled every gap, so the modern table has no empty blocks. _Source: Siyavula Natural Sciences Grade 7 (CC BY 4.0), Comparing Mendeleev's table with the modern version of the Periodic Table_