Development of the Periodic Table

By the time of 1869, Mendeleev wanted to discover and learn more about chemistry, it was also the time where he thoughts the known elements of that time can be logically arranged.
He wrote all of the 65 known elements on a piece of paper like flashcards, he labelled some of there properties like atomic weight.
He realized there was a pattern as he looked at the atomic weight, he spent countless hours arranging it until he fell asleep one night.
That night he had a dream which came up with the perfect arrangement of the elements and introduced the periodic table to the world of chemistry.

Dimitri Mendeleev

Henry Moseley was celebrating in 26th birthday in 1913, he looked at how Mendeleev's periodic table was then over 40 years old, until he realized that it had a flaw, and that the elements cannot be arranged just with their atomic weight.