defects of Mendeleev periodic table
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1. Anomalous pairs
The following pairs of elements did not follow Mendeleev’s Principles:
Cobalt (58.9) with higher atomic mass precedes Nickel (58.6) in the periodic table.
Tellurium (127.6) with higher atomic mass than Iodine (126.9) is also placed before it in the periodic table.
2. Position of Isotopes
The isotopes of an element are atoms of that element having similar chemical properties but different atomic masses. According to Mendeleev’s Periodic Law, isotopes of an element must be given separate places in the periodic table because they have different atomic masses.
3. Grouping of chemically dissimilar elements
Elements such as Copper and Silver bear no resemblance with the alkali metals such as Lithium and Sodium etc., but they have been placed together in the first group.
4. Separation of chemically similar elements
Elements which are chemically similar such as Gold and Platinum have been placed in separate groups.
5. Electronic arrangement
It does not explain the electronic arrangement of elements.
6. The position of hydrogen
Hydrogen was not given a fixed position. It was considered in Group IA as well as in Group VIIA, because it forms a positive ion as in HCl but also a negative ion as in NaH
drawback' s
it was not valid for elements that had atomic mass higher than calcium
when more elements were discovered such as noble gas then they could not accommodate in this table
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