Mendeleev predicted the existence of an element he called eka-silicon, later identified as:

1. Mendeleev left gaps in his table for unknown elements. 2. He named them by adding 'eka' (Sanskrit for 'one') to a known element's name. 3. Eka-aluminium was later identified as gallium. 4. Eka-boron turned out to be scandium. 5. Eka-silicon was confirmed as germanium when Winkler discovered it in 1886. 6. So the answer is germanium. _Source: syllabus Science Ch 5 'Periodic Classification of Elements'_