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WillDabFoYoMa
In the modern periodic table, the elements are arranged in order of increasing atomic number. (In Mendeleev's table, they were arranged in order of increasing atomic weight). More elements are in the modern periodic table which have been discovered since Mendeleev's time.
roytucker
Mendeleev ordered the elements in his periodic table according the atomic mass of the element, whereas the modern day periodic table has the elements ordered according to their Atomic number (or the Proton Number).