Eli5: How did Mendeleev predict the existence of elements that had not been discovered yet?

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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
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The periodic table is called “periodic” because it has regular repeating patterns. Mendeleev recognized the patterns and assumed (correctly) that they ought to continue even in places where we hadn’t found the element that would fill that “slot” yet.

For example, if I tell you I have Heads-Tails-Heads-Tails-H-T-H-?-?-T-H-T-H-T you can probably guess there’s a missing “H-T” in there because of the pattern.

It turns out the patterns are caused by the number of electrons in different shells that quantum mechanics allows (kind of like orbits, but more complicated) but Mendeleev didn’t know that.

Basically, the periodic table is set up in such a way that columns of elements are similar and there are repeating patterns of characteristics and properties, so he knew that there were elements missing when the pattern had gaps. He was even able to predict the properties of the missing elements based on the table

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.

Imagine for a moment that we haven’t discovered all the numbers yet.

We know there’s: …..12, 13, 14, 15, 16…..

We know there’s …..22, 23, 24, 25, 26…….

We know there’s ….32, 33, 34, (is there something here?), 36….

We know there’s…..42, 43, 44, 45, 46…..

Based on the clearly established patterns before and after, we can make a very confident assumption that there’s a “35” in existence that we just haven’t found yet. We leave a gap to account for this extremely likely possibility that someone in the future will discover this “35” number.