Answer to Question #187055 in Organic Chemistry for Semi

Question

Why 5d is less in energy than 4f in lanthanides?

Answer 1

Orbitals fill in order of energy. So 5D fills before 4F in some cases simply because the 5D energy levels are lower than the 4F levels for some. This does not occur for all the lanthanides.The nuclear charge is insufficient to contract the 4F orbitals and lower their energy well below the 5D.