Final answer:
In the early 1800s, elements were classified by atomic masses, and Mendeleev introduced gaps for undiscovered elements, predicting their properties.
Explanation:
In the early 1800s, elements were classified primarily based on atomic masses, where elements with similar properties were arranged together.
Mendeleev made two significant changes to the early periodic tables: he arranged elements based on atomic masses but left gaps for undiscovered elements, and he predicted properties of these missing elements that matched those of later-discovered elements, solidifying the acceptance of his periodic table.
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