SAQ : Mendeleev violated his periodic law and placed lower atomic weight 39K after  40Ar having higher atomic weight. How this placement is justified in modern periodic table?

Question

Mendeleev violated his periodic law and placed a lower atomic weight of ³⁹K after  ⁴⁰Ar had a higher atomic weight. How this placement is justified in the modern periodic table?

Answer

In the modern periodic table, ³⁹K is placed before ⁴⁰Ar because it has a lower atomic number and belongs to the same group (Group 1A) as other alkali metals. This reflects the more important role of atomic number in determining the placement of elements in the periodic table, as compared to atomic weight.