Question

which element in lanthanides is acception why?

Answer 1

Since the list is long I won’t be able to provide explanation to all though u may find explanation for copper and chromium in my answer to question : Himanshu Ranjan's answer to Why does 4s1 come after 3d5 in chromium ion's electronic configuration?

List of anomalous electronic configurations are:

Chromium :- [Ar] 3d5 4s1

Copper :- [Ar] 3d10 4s1

Niobium :- [Kr] 4d4 5s1

Molybdenum :— [Kr] 4d5 5s1

Ruthenium :— [Kr] 4d7 5s1

Rhodium :— [Kr] 4d8 5s1

Palladium :- [Kr] 4d10 5s0

Silver :— [Kr] 4d10 5s1

Lanthanum :— [Xe] 5d1 6s2

Cerium :— [Xe] 4f1 5d1 6s2

Gadolinium :— [Xe] 4f7 5d1 6s2

Platinum :— [Xe] 4f14 5d9 6s1

Gold :— [Xe] 4f14 5d10 6s1

Actinium :— [Rn] 6d1 7s2

Thorium:— [Rn] 6d2 7s2

Protactinium :— [Rn] 5f2 6d1 7s2

Uranium :— [Rn] 5f3 6d1 7s2

Neptunium :— [Rn] 5f4 6d1 7s2

Curium :— [Rn] 5f7 6d1 7s2