Several scientists noticed patterns independently. Johann Dobereiner in 1829 grouped elements into triads, where the middle element's weight averaged the other two. John Newlands proposed that every eighth element repeated similar properties.

Lothar Meyer built a table based on physical properties. But Dmitri Mendeleev in 1869 made the boldest move: he arranged 63 elements by weight and left gaps for ones not yet discovered.

Those gaps were a bet. When the missing elements turned up with the properties he had predicted, the periodic table went from a clever guess to one of science's most powerful frameworks.