Modification in Mendeleev's Periodic Table: With the adoption of atomic number as the basis of classification of elements, some of the anomalies in the Mendeleev's Periodic Table now disappeared. These are :
(i) Position of Hydrogen: The atomic number of hydrogen is 1 and hence, it should occupy the first position in group 1 of the Periodic Table. But because of its chemical similarity with elements of group 1 and group 17, tine position of hydrogen in the periodic table is still not confirmed.
(ii) Position of Isotopes : As the isotopes of an element have the same atomic number, they should occupy the same place in the periodic table.
(iii) Position of Anomalous Pairs: The position of anomalous pairs of elements i.e., pants of elements in which the element with the higher atomic mass proceeds the element with lower atomic mass, is also decided. When these elements are arranged according to their atomic number, they occupy their right places in the periodic table.
