The early attempts to classify elements were made on the basis of atomic masses of the elements. The modern basis is the atomic number. This is better because now it is established that an atomic number of an element is the fundamental property. Further, there are certain anomalies in Mendeleev's periodic table such as the position of isotopes and misplacements of certain elements, e.g. Argon is placed (Atomic mass - 40, atomic number= 18) before potassium in Mendeleev's periodic table (Atomic mass =39, At. no = 19).
