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Limitations of Mendeleevs Periodic Table

Explanation of Limitations of Mendeleev's Periodic Table


In the Modern periodic table, elements are arranged in increasing order of their atomic numbers. This periodic table nicely explains the three main limitations of the Mendeleev's periodic table as discussed below :


1. Position of isotopes. All the isotopes of an element have the same atomic number.Therefore,they can be placed at one place in the same group of the periodic table.

Thus, all the three isotopes of carbon, i.e ., C-12, C-13 and C-14 are placed at the same position as that of carbon (group 14) in the periodic table. Likewise all the three isotopes of hydrogen, i.e ., protium(₁¹H), deuterium ( ²₁H  or D) and tritium ( ³₁H or T) are placed at the same place as that of hydrogen (group 1) in the Modern periodic table.


2. Anomalous position of some pairs of elements. In Mendeleev's periodic table, cobalt with slightly higher atomic mass (58.93 u) was placed before nickel with slightly lower atomic mass (58-71 u). This limitation has been removed in the Modern periodic table as follows.

The atomic number of cobalt is 27 while that of nickel is 28. Since in the Modern periodic table, elements are arranged in increasing order of their atomic numbers, therefore, cobalt with lower atomic number comes first and nickel with higher atomic number comes later. Similarly, the atomic number of tellurium (Te) is 52 while that of iodine (I) is 53. Therefore, tellurium is placed before iodine even though the atomic mass of tellurium is higher (127-6 u) than that of iodine (126-9 u).

Likewise atomic number of argon is 18 while that of potassium is 19, therefore, argon is placed before potassium even though the atomic mass of argon (39.9 u) is higher than that of potassium (39.1).


3. Uncertainty in prediction of new elements. Since atomic masses do not increase in a regular manner in going from one element to the next, therefore, in Mendeleev's periodic table, it was not possible to predict as to how many new elements could be discovered between two known elements. This limitation has been removed in the Modern periodic table because atomic numbers or proton numbers increase by one in going from one element to the next. Therefore, the number of new elements to be discovered in between any two known elements is equal to the difference in their atomic numbers or proton numbers.