Question

What did mendeleev contribute to the modern periodic table?

Answer 1

He basically invented it. He noticed some patterns in how they lined up and was able to predict elements based on their weights and properties by looking across periods and down columns. So for example, when you notice all the Noble gasses are lined up? That's all him. It's incredibly useful, and quantum mechanics actually allows you to derive why the periodic table is like that by looking at the orbitals and how reactive the elements are, but that's another thing to worry about another time.