Question

why the electron configuration of some inner transition element is nS^2,4f^7,5d^1 , as aufbau principle ns2,4f8 .. why electron occupies d sublevel?

Answer 1

Since 4f (4+3=7) and 5d (5+2=7) almost have same energy gap.Hence, first it will go in d-subshell and then again after filling it it will start pairing up.