How did Dmitri Mendeleev contribute to the atomic number?
Atomic number was discovered well after Mendeleev published his first periodic table in the 1860s.
Atomic number was not discovered until after Mendeleev's death in 1907.
So how did Mendeleev contribute to atomic number? Mendeleev's table arranged the elements according to their atomic mass and their chemical properties. Some elements seemed to be somewhat out of position.
When Mosely discovered atomic number in 1913, he realized that re-arranging the elements based on atomic number rather than atomic mass would fix the problems from Mendeleev's table.