The brilliance of Mendelvee in developing the table was to arrange the elements that he knew according to their properties. He left gaps where he felt new elements would be found. These gaps were based on the assumption that missing elements would have properties similar to the elements in the table above and below the gap.
The modern periodic table is arranged according to the atomic number, or the number of protons. However the arrangement of the elements is still organized according to families of elements. Any vertical column of the periodic table is a family of element. The elements in a family have similar properties and numbers of valance electrons.