What is the main difference between the modern and earlier periodic table?

The modern periodic table has elements order by their atomic number I.e. the number of protons in their nucleus. Very early periodic tables had elements ordered by atomic weight (note that this is not relative atomic mass). Mendeleev then went on to group elements according to their properties. This somewhat matched with what Newlands had worked out with the “Law of Octaves” where he noticed a pattern in properties of the elements which differed in atomic weight by 7.