how were the limitations of mendeleevs periodic table rectified?

Look at first the limitations:

Position of hydrogen : Hydrogen is placed in Group IA. However, it actually resembles the elements of Group-IA (alkali metals ) as well as the elements of Group VII-A (halogens ). Thus, the position of hydrogen in the periodic table is not clear.

Position of isotopes : On the basis of atomic weight , various isotopes of the same elements should be assigned different places in the periodic table. Mendeleev could not provide separate places for isotopes.

Disimilar elements placed together : Noble metals like Cu, Ag and Au are placed along with chemically dissimilar alkali metals in Group I . Similarly, Mn possessing very few similarities with halogens have been placed in VII group.

Similar elements separated In Mendeleev 's periodic table, certain chemically similar elements such as copper and mercury ; gold and platinum have been placed in different groups.

Anomalous pairs : In the Mendeleev 's Table based on atomic weight, the positions of certain pairs , e.g. Argon( at. wt = 39.94) and potassium ( at. wt = 39.1) : Cobalt( at wt =58.93 ) and nickel ( at wt = 58.71 ) ; Tellurium at wt = 127.60) and iodine (atomic weight = 126.90 ) would be reversed. In other words, certain pairs of elements are misfit in the periodic table, if atomic weight is the basis of classification.

Rectification :
1) Hydrogen is placed in group-1 according to number of valence electron but still its position is debatable.
2) Position of isotopes is solved by giving place alongside their base element. Because modern periodic table is based on atomic number and it does not effected by having different mass for same element.
3) By arranging the elements according to atomic number and by properly arranging them (not in sub groups) in different groups, the problem of dissimilar and similar atom is already solved.
4) Problem of anomalous pair is already solved as K is higher in atomic number than Ar, Ni is higher in atomic number than Co and so on.

 

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1) Hydrogen was placed in the first group - alkali metals.
2) Cobalt was assigned Atomic No. 27 and Nickel was given 28. So now Cobalt was placed where it belonged and so was Nickel regardless of mass.
3) Isotopes were given place alongside their base element.
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# shikhar sachdev .....
hydrogen was placed in a separate group .. not with alkali metals

 
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  1. by using atomic no. instead of atomic mass
  2. by adding 18th group
  3. by giving separate groups to the sub groups of mendeleev's table
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#Divyanshi please verify. Hydrogen is placed in group 1 :)
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