i.
Hydrogen
was placed in group 1 and in group 17 as it resembles alkali metals as well as
halogens. Therefore, no fixed position could be given to hydrogen in
Mendeleev’s periodic table. But in Modern
periodic table it is placed in group 1.
ii.
Mendeleev
arranged the elements in their increasing order of atomic masses. But some
elements with higher atomic masses are placed before those having lower atomic
masses. E.g. Cobalt (Co) with atomic mass 58.93 is placed before Nickel (Ni)
having atomic mass 58.71. But in Modern periodic table the elements were
arranged on the basis of the atomic number of elements. Thus atomic number of
Co is 27 and that of Ni is 28. So Nickel is placed after Cobalt.
iii.
Though
isotopes of the same elements have different atomic masses, they cannot be
placed separately in the periodic tale. Since isotopes are chemically similar,
they had to be given the same position in the periodic table. Isotopes occupy
the same position in the Modern periodic table.
iv.
In
Modern periodic table elements are classified according to their electronic
configuration into different blocks. Thus, various anomalies (irregularity)
which were seen in Mendeleev’s table due to the arrangement according to atomic
masses were overcome by the Modern Periodic table.
