When Dmitri Mendeleev created the Periodic Table of the Elements, he decided against arranging them by their atomic numbers based on his correct assumption that those numbers were not totally accurate. When he first began to arrange the 60 known elements in the 1860s, he knew that new elements would eventually be discovered there are now over 100 known elements and that the atomic weights of the elements known at that time had been calculated incorrectly.
The Russian chemist, known as the father of the periodic table, saw that some of the elements appeared in the wrong place when arranged by their atomic weights. For example, he saw that a reactive non metal element would be followed by a highly reactive light metal and then be followed by a less reactive light metal. Mendeleev came to the conclusion that by following what were the then assumed atomic weights, the elements appeared in places where their properties did not match those of their neighbors.