Mendeleev's Periodic Table Merits and De-merits

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 Important Contributions (Merits) of Mendeleev's Periodic Table

Mendeleev's periodic table was one of the greatest achievements in the development of chemistry. Some of the important contributions of his periodic table are:

 1. Systematic study of the elements: The Mendeleev's periodic table simplified the study of chemistry of elements. Knowing the properties of one element in a group, the properties of other elements in the group can be easily guessed. Thus, it became very useful in studying and remembering the properties of a large number of elements.

2. Correction of atomic masses: The Mendeleev periodic table helped in correcting the atomic masses of some elements based on their positions in the table. For example, atomic mass of beryllium was corrected from 13.6 to 9, Similarly, with the help of this table, atomic masses of indium, gold, platinum etc. were corrected.

3. Prediction of new elements: At the time of Mendeleev, only 56 elements were known. While arranging these elements, he left some gaps. These Rape represented the undiscovered elements. Mendeleev predicted the properties of these undiscovered elements on the basis of their positions. For example he predicted the properties of gallium (e k a-aluminium) and germanium (e k a-silicon) which were discovered later. The observed properties of these elements were found to be similar to those predicted by Mendeleev.

 


Defects (De-Merits)of Mendeleev's Periodic Table In spite of many advantages, the Mendeleev's periodic table had certain defects also. Some of these are given below:


1. Position of hydrogen:  Hydrogen is placed in group I. However, it resembles the elements of group I (alkali metals) as well as the elements of group V I I A (halogens). Therefore, the position of hydrogen in the periodic table is not correctly defined.

2. Anomalous pairs:  In certain pairs of elements, the increasing order of atomic masses was not obeyed. In these cases, Mendeleev placed elements according to similarities in their properties and not in increasing order of their atomic masses. For example, argon (A r, atomic mass 39.9) is placed before potassium (K, atomic mass 39.1). Similarly, cobalt (Co, atomic mass 58.9) is placed before nickel (N i, atomic mass 58.6) and tellurium (T e, atomic mass 127.6) is placed before iodine (I. atomic mass 126.9). These positions were not justified.

3. Position of isotopes: Isotopes are the atoms of the same element having different atomic masses but same atomic number. Therefore, according to Mendeleev's classification, these should be placed at different places depending upon their atomic masses. For example, isotopes of hydrogen with atomic masses 1, 2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table.

4. Some similar elements are separated and dissimilar elements are grouped together. In the Mendeleev's periodic table, some similar elements were placed in different groups while some dissimilar elements had been grouped together. For example copper and mercury resembled in their properties they had been placed in different groups. At the time, elements of group I A such as L i, Na and K grouped with copper (Cu), silver (A g) and gold (A u) though their properties are quite different. 

5. Cause of periodicity. Mendeleev did explain the cause of periodicity among the elements. 

6. Position of  lan t h an o i ds (or lanthanide and actinoids (or actinides). The fourteen element following lanthanum (known as lan t h an o i ds, free atomic number 58-71) and the fourteen elements following actinium (known as actinoids, from at number 90—103) have not been given separate plants in Mendeleev's table.

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