For Mendeleev the rare earths were a complete nightmare because he didn't know where to put them. He couldn't fit them in the table..! Five of them had been found by the time he was building the table, and so he... stuck them in somewhere where things went 3+, and then went Uh? and... left it at that. ...[T]his was a real problem, because no one knew where these building blocks went into the periodic table. ...[I]t wasn't ...until Moseley had established what atomic number was, that things began to fit together... and suddenly they realized that there couldn't be more than 14... [T]hen as the quantum mechanics rules came through it became clear... that... you'd found the hole. There was the gap... in Promethium, and so that became a target.
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About Dmitri Mendeleev
Dmitri Mendeleev was a 19th-century Russian chemist. Dmitri Ivanovich Mendeleev was a Russian chemist known for formulating the periodic law and creating a version of the periodic table of elements. He used the periodic law not only to correct the then-accepted properties of some known elements, such as the valence and atomic weight of uranium, but also to predict the properties of three elements that were yet to be discovered. Read more on Wikipedia →