Question

Ground state electronic configurations of gadolinium and lutetium are different than expected. Explain why?

Answer 1

Element	Symbol	Atomic Number	Expected (ground state)	Observed (ground state)
Gadolinium	Gd	64	[Xe] 4f8 6s2	[Xe] 4f7 5d1 6s2
Lutetium	Lu	71	[Xe] 4f14 5d1 6s2	[Xe] 4f14 5d1 6s2

Gadolinium and lutetium show different ground-state electronic configurations because the 5d and 4f-orbitals are nearly of the same energy due to which the next electron goes in 5d-orbital instead of 4f-orbital. This retains the extra stability achieved when the 4f orbital is half-filled and completely filled respectively. Thus, 5d orbital contains one electron in case of gadolinium and lutetium.