"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 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 , and so that became a target."