Question

Was there any defect in Mendeleev's periodic table?

Is there any defect in the modern periodic table? What are those?

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.

Following are the defects of the modern periodic table:

• Position of hydrogen still dicey. It is not fixed till now.
• Position of lanthanides and actinides has not been given inside the main body of periodic table.
• It does not reflect the exact distribution of electrons of some of transition and inner transition elements.