Ans: option (B).
How barium has higher I.E. than potassium?
Q.19. Decreasing ionization potential for K, Ca & Ba is
(A) Ba > K > Ca
(B) Ca > Ba > K
(C) K > Ba > Ca
(D) K > Ca > Ba

Dear Student,
Let us consider the electronic configuration of cations generated after removal of one electron from each K, Ca and Ba.



Electronic Configuration
K 3s2 3p64s1
Ca 3s2 3p6 4s2
Ba 5s2 5p6 6s2
 As we can see that, K will have completely filled outermost shell after removal of one electron. Therefore, removal of the electron will require less amount of energy. Thus, among the given elements K will have the lowest ionization energy.
Among Ca and Ba, Ca will have higher ionization energy, because ionization energy decreases on moving down the group.

Thus, the correct order of increasing second ionization energy is  Ca> Ba >  K.

Hope this information will clear your doubts regarding ionization energy.

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