On what fundamental property did Mendeleev base the arrangement of elements in his periodic table?

1. Mendeleev developed his table between 1869 and 1872. 2. He treated atomic mass as the standard fundamental property. 3. He arranged the 63 known elements in increasing order of atomic mass. 4. Atomic number was used only later, by the modern periodic law. _Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 2 "Periodic Classification of Elements", p.27_