Electron Configuration of Nb

[RosemarieDiPentino3B]

Why is the electron configuration of Niobium in the ground state 4d^4 5s^1 instead of 4d^3 5s^2?

[Patrick Chin 1F]

This is one of the many exceptions on the periodic table, because the energy gap between the 4d and 5s orbitals is small enough for the electron to be pulled from the 5s orbital due to electron repulsion, sort of like Hund's rule except applied across orbitals.