All of the elements are arranged on the periodic table.
There are some key factors of the arrangement of the periodic table:
The horizontal rows are called periods
The vertical columns are called groups
The elements are arranged in the order of increasing atomic number/proton number
Elements that are in the same group have similar properties.
Mendeleev made the table two-dimensional rather than a list. He arranged the elements in order of atomic mass but had the Newlands Octave elements underneath each other. The vertical columns are called groups and the horizontal rows are called periods.
Some elements had not been discovered at the time of Mendeleev writing up his table therefor he left gaps to indicate where he thought elements should be. Later chemists discovered elements to fit in the right places and they were amazed to find that the properties were very much like those Mendeleev had predicted.