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We follow the modern periodic table which was given by Mosely and it is based on Atomic number.It is because every element have unique atomic number.
If we follow Mendeleev's periodic classification, if was arranged on the basic of Atomic masses but later, scientist found element which have same atomic mass but having different Atomic number(called isobar). So it was his limitation.
So atomic mass is right way to arrange elements, and the element are arranged in increasing order of atomic number and the periodic table has 7 periods and 18 groups.