Question

Defect of Mendeleeff's original table?

Answer 1

Mendeleev's periodic table suffered defects as follows:

The position of hydrogen was not correctly defined. It was placed in group I although it resembles both the group I elements - the alkali metals and the group VII elements-the halogens, in their properties.

In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses. Thus, the position of these elements was not justified e.g. cobalt (atomic mass 58.9) was placed before nickel (atomic mass 58.6).

Isotopes were not given separate places in the periodic table although Mendeleev's classification is based on the atomic masses.

Some similar elements were grouped separately while some dissimilar elements were grouped together. For example copper and mercury are similar in their properties but were placed separately. Copper was placed in group I although it did not resemble the elements of this group.

Mendeleev could not explain the cause of periodicity in the elements.

The position for lanthanides and actinides were not included in this table.

Answer 2

When Mendeleev’s’ periodic table was re-defined on the basis of atomic number, groups are sub-divided in A and B, and zero group was inserted then the problem of anomalous pairs, position of isotopes were eliminated. The position of rare earth elements was still not shorted out. This was the main drawback which was corrected in modern periodic table, when elements were arranged on the basis of electronic configuration in four blocks i.e. s, p ,d and f block.

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