In Dmitri mendeleev's periodic table elements were placed in order of?

In Dmitri Mendeleev's Periodic Table, elements were arranged in order of increasing Atomic Mass. Mendeleev noticed that when elements were organized this way, they exhibited periodic trends in their chemical properties. This arrangement allowed him to predict the existence and properties of undiscovered elements by leaving gaps in the table. His work laid the foundation for the modern periodic table, which is now organized by atomic number instead.