Dmitri Mendeleev was convinced he had uncovered the chemical “order” of the universe, but he had two problems.
First, there were gaps in the table-places where elements were needed to complete the group.
Mendeleyev simply said these were elements that had not yet been discovered.
He said they would be discovered soon, and he predicted the atomic weights and the properties those elements would have.
Six years later, the French chemist Paul-Emile Lecoq de Boisbaudran discovered the first of these missing elements.
Mendeleyev called the element gallium, from the Latin gallia meaning “France”, and it had almost all the properties Mendeleyev had predicted. The other missing elements would soon be discovered also.
The other problem made Mendeleyev doubt his entire arrangement. Two elements, tellurium and gold, seemed to be in the wrong place.
Based on their atomic weights, they were in groups with different properties.
Mendeleyev made another bold decision: their atomic weights must be wrong. He placed them in the group he thought they belonged in and estimated what their correct atomic weights should be.
Further experiments proved him to be right again: their atomic weights had been miscalculated.