How did Mendeleev organize the elements to create a periodic table

Mendeleev arranged the elements known at that time according to atomic mass. The elements were arranged in rows called periods, in increasing order. Each row had eight elements. He also arranged them accordingly in columns, where the elements in each column, called groups, had similar properties.

He left some blanks on his table because he predicted that the big gaps between some atomic mass' of elements indicated that there were other elements yet to be discovered that would fill in that gap. He was able to predict the atomic weights of some of the elements in between.