Give any two defects of Mendeleev’s periodic table which has been removed in modern periodic table. How were they removed?
Answers
Explanation:
In Mendeleev's periodic table
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.
2)Isotopes were not given separate places in the periodic table although Mendeleev's classification is based on the atomic masses.
Hope this helps you.
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.