Hydrogen as 1 electron in their outermost shell and alkali metals also have 1 electron in the outermost shell .
Therefore many properties of hydrogen are same to those alkali metals.
Hence , hydrogen can be placed along the group of alkali metal in the Mendeleev's Periodic Table.
{ isotopes : those element whose atomic number is same but atomic mass is different . }
The Mendeleev's Periodic table was based on the increasing order of atomic masses and isotopes of the elements have different atomic masses , therefore they should be placed at different position , but due to the same chemical properties they are placed in the same position.
Thus , isotopes of the element posed a challenge to Mendeleev's Periodic Table.
In the Mendeleev's Periodic Table , elements placed in a group having similar chemical properties . But there is no similarity in the properties of elements in two sub groups.
For example , Li , Na , K , Rb , Cs present in group I A are quit different from Cu , Ag , Au in group II B .
In the Mendeleev's Periodic Table , elements are arranged in increasing atomic masses . But the atomic masses do not increase in regular manner in going from one element to the next .
Further , the difference in atomic masses between two successive elements is small among lighter elements and large in heavier elements .
Therefore , it is not possible to predict how many new elements could be discovered between two known elements especially among heavier elements.
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Limitations Of Mendeleev's Periodic Table
✦ Anomalous position of Hydrogen
Hydrogen as 1 electron in their outermost shell and alkali metals also have 1 electron in the outermost shell .
Therefore many properties of hydrogen are same to those alkali metals.
Hence , hydrogen can be placed along the group of alkali metal in the Mendeleev's Periodic Table.
✦ Position of isotopes
{ isotopes : those element whose atomic number is same but atomic mass is different . }
The Mendeleev's Periodic table was based on the increasing order of atomic masses and isotopes of the elements have different atomic masses , therefore they should be placed at different position , but due to the same chemical properties they are placed in the same position.
Thus , isotopes of the element posed a challenge to Mendeleev's Periodic Table.
✦ Different groups of elements within sub groups
In the Mendeleev's Periodic Table , elements placed in a group having similar chemical properties . But there is no similarity in the properties of elements in two sub groups.
For example , Li , Na , K , Rb , Cs present in group I A are quit different from Cu , Ag , Au in group II B .
✦Uncertainity in prediction of new elements
In the Mendeleev's Periodic Table , elements are arranged in increasing atomic masses . But the atomic masses do not increase in regular manner in going from one element to the next .
Further , the difference in atomic masses between two successive elements is small among lighter elements and large in heavier elements .
Therefore , it is not possible to predict how many new elements could be discovered between two known elements especially among heavier elements.
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❁ Extra Shots❁
☞ Mendeleev is also called as father of Modern Periodic Table .
☞ He was the first person to arranged all the 63 elements that were discovered that time into groups { vertical columns } and periods { horizontal rows } .
☞ The noble gases like Ar { Argon } , Ne { Neon } were not known when Mendeleev give his periodic table . These gases were discovered much later by one of the student of Mendeleev , Henry Moseley.
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