They have used added new elements to the table .
Explanation:
As new elements are discovered chemists add them to the period table leading to the periodic table undergoing continuous modifications. When the first periodic table in the world was formed, very few elements were known to humankind. Earlier periodic tables thus contain lesser number of elements than the modern ones.
Mendeleev's periodic table contained 63 elements arranged in the increasing order of their atomic masses. Modern periodic tables have far more number then the older ones like that of Mendeleev's and the elements are arranged in the order of their atomic numbers.
