Before subatomic particles were discovered, scientists ordered the elements by their atomic weight.
These early periodic tables were incomplete and many elements were in the wrong place.
This was because of the atomic weight order.
Mendeleev left gaps in his periodic table for undiscovered elements.
However, he also used the atomic weight of elements to order them, meaning his periodic tables were still not very accurate.
As more elements were discovered, they were placed in the gaps of Mendeleev's periodic table.
Eventually, isotopes were discovered, which explained why grouping by atomic weight was not always correct.
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