Question

THE ELECTRONIC CONFIGURATIONS OF EU(ATOMIC NO.63),Gd(ATOMIC NO.64),AND Tb(ATOMIC NO.65) IS [Xe]4f76s2,[Xe]4f75d16s2,and [Xe]4f9 6s2.MY QUESTION IS WHY IN Gd [atomic no.64]instead of placing one electron in 4f7 it is placed in 5d orbital while in Tb it is placed in 4f and the 5d is not taken in this case?

Answer 1

Dear Student,

Electronic configuration of Gd = [Xe] 4f^​7 5d¹ 6s²
Electronic configuration of Tb =[Xe] 4f⁹ 6s²

In Gd, one electron instead goes to 5d instead of 4f. This is because 4f and 5d orbital in Gd are about the same energy level and Gd atom has a tendency to retain the configuration with half filled 4f level which are more stable. Since, this is not in the case of Tb, therefore it forms the normal lanthande trend and has the normal electronic configuration.