Defects And Improvements Of Mendeleev's Periodic Table

Defects in Mendeleev ‘s  periodic Table
In spite of its useful role in the study of chemistry. Mendeleev’s Table possessed many drawbacks. Some of these drawbacks are:
Position of Hydrogen
Hydrogen is placed in Group  IA. However, it actually resembles the elements of Group –IA (Alkali earth metals) as well as the elements of Group VII-A (Halogens). Thus, the position of hydrogen in the periodic table in not clear.
Position of Isotopes
On the basis of atomic weight, various isotopes of the same elements should be assigned different places in the periodic table. Mendeleev could not provide separate places for isotopes
Position of Lanthanides and Actinides
Fourteen elements following Lanthanum and the fourteen elements following Actinium have not been provide separate proper places in the Mendeleev Periodic table, rather they have been placed in two rows at the bottom of the table.
Dissimilar elements placed together
Noble metals like Cu, Ag and Au are placed along with chemically dissimilar alkali metals in Group I. Similarly, Mn possessing very few similarities with halogens have been placed in VII group.
Similar elements Separated
In Mendeleev’s periodic table, certain chemically similar elements such as copper and mercury ; gold and platinum have been placed in different groups.
Anomalous pairs
In the Mendeleev’s Table based on atomic weight , the position of certain pairs like Argon and Potassium : Cobalt and nickel ; Tellurium  and Iodine would be reserved. In an other words, certain pairs of elements are misfit in the periodic table, if atomic weight is the basis of classification.
Atomic Number
The elements were arranged in ascending order of their atomic instead of their atomic mass.
Extra Group
For the arrangement of Noble gases, one extra group was added at the extreme right of the periodic table. These gases had not been discovered in Mendeleev’s time.
Sub Group
In Mendeleev’s Periodic table, the element like Be, Mg Ca, Sr, Ba and Zn Hg were placed in single vertical group although they possess different properties. This confusion was removed by dividing the elements in two types of vertical group A and B.
Position of Isotopes
If the elements are arranged in the order of their increasing atomic masses, it is not possible to accommodate a large number of isotopes. This problem solved by arranging elements in ascending order of their atomic numbers.
 Regular Arrangement of Pairs
Anomalous pairing can be rectified by arranging them in increasing order of their atomic numbers. Like Ar should be placed before K. similarly Co should be placed before Ni and Te should be placed before I.