Re: Why does electron pathway 5d^1 fill before 4f^1

Hi Mike.

As you know, electrons enter the lowest available empty orbital.    The 4f 
and 5d energy levels are very close and there is a considerable amount of 
uncertainty in some of the electron configurations.  The orbital energies 
of the elements are measure using photoelectron spectroscopy techniques.

Usually, orbitals of the same quantum number n, such as the 2s and 2p 
orbitals have similar energies and are well separated from orbitals of 
different n.  This confirms the shell structure of many-electron atoms.  
Note, however, 3d orbitals lie well abovethe 3s and 3p orbitals and only 
slightly above the 4s orbitals.

The 4d, 5d, and 4f orbitals have very similar energies.  There is great 
graph in "Principles of Modern Chemistry, 3rd ed." by David Oxtoby & Norman 
Nachtrieb, Figure 13-32 of atomic number vs. energy of orbitals that 
depicts the energy similarity very well.  The energy lines overlap showing 
their similar energies and explains why you would fill the 5d first.

-Bonnie