He intentionally left gaps between the already discovered elements, so that later on those gaps would be filled with new elements that he knew existed, but haven’t been discovered yet. How did he know that?
In: Chemistry
He was standing on the shoulders of scientists who came before him. In the early 1800’s, John Dalton figured out a way to measure the atomic weights of different substances. William Prout figured out that atomic weights seemed to be multiples of the atomic weight of hydrogen. In the 1860’s, John Newlands realized that if you listed elements in order by atomic weight, you had common properties in every multiple of 8 (e.g, atomic number 2, Helium, had common properties with number 10, Neon and number 18, Argon.)
So because other scientists had done all this work, it was pretty clear that there must be gaps. You know there’s an element with 30 protons – zinc – and you know there’s an element with 33 protons – arsenic – so logic suggests there must be a 31 and a 32. These are gallium and germanium, and they weren’t discovered for another decade or so after Mendeleev published his table.
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