CBSE Class 10 Answered
How were the following defects in the original Mendeleev's Periodic Table resolved in the modified Mendeleev's Periodic Table?
(i) Problem of Anomalous Pairs
(ii) Position of Rare Earths
(iii) Position of Isotopes
Asked by vatsalchoudhary41 | 24 Feb, 2016, 06:20: PM
Expert Answer
The modified Mendeleev's Periodic Table is known as Modern Periodic Table. In this, the elements are arranged according to their atomic numbers.
It resolves the defects of Mendeleev's Periodic Table as follows:
(i) Problem of Anomalous Pairs:
In original Mendeleev’s Periodic Table, the elements were arranged on the basis of their atomic masses. There were three pairs of elements in which atomic mass of the preceding element was higher than that of the following element. It is shown below.
However, with the modern classification of elements on the basis of atomic numbers, this anomaly was removed.
(ii) Position of Rare Earths:
A group of fifteen elements, after barium are placed in group III A. Thus, these elements were not given proper places in the periodic table. This problem has been removed in the Modern Periodic Table, by placing these elements in a separate row below the modern periodic table.
(iii) Position of Isotopes:
Mendeleev’s original periodic table is silent about isotopes. However, with the modern classification on the basis of atomic numbers, this defect is removed as all isotopes have the same atomic number and hence same place for a particular element. Only most stable isotope of an element is given the position in the modern Periodic Table.
Answered by Hanisha Vyas | 25 Feb, 2016, 01:00: PM
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