Question

Why does niobium have a $ {d^4}{s^1} $ electron configuration but vanadium has a $ {d^3}{s^2} $ electron configuration?

Answer 1

Hint: The given two elements Nb and V belong to the transition metal series of the periodic table. This question can be answered based on the size of the valence orbitals, the difference in the energies between orbitals and the electron repulsions.

Complete Step By Step Answer:
First, let us know the energy difference between the valence orbitals in the respective elements Nb and V. Vanadium belongs to the first transition series and Nb belongs to the second transitions metal series.
The energy gap between 3d and 4s orbital in Vanadium is 2.79eV and that of 4d and 5s of Niobium is 1.64eV. Thus, we can say that the valence orbitals of Nb are very close to each other. Because of this closeness of the orbitals in Nb, electrons can easily jump from 4d to 5s. As a result, we might expect that, based on the energy the fourth valence electron may enter into the 5s orbital instead of 4d during the filling of electrons, to reduce the electron repulsion in 4d of Nb.
Based on the orbital size, we can say that the size of 4d is bigger than 3d and 5s is bigger than 4s. Because of the bigger orbital size, the electron repulsion can be tackled more efficiently if the electrons enter the 5s orbital than the 4d in Niobium.
Hence we can conclude that having bigger orbitals reduces the electron repulsions and makes it more favorable to continue filling the d orbital with the fourth valence electron in Niobium. Also, the 4d orbital being close to the 5s orbital makes it more favorable for the fourth electron to be placed in the s orbital than the d.
Because of this reason niobium has a $ {d^4}{s^1} $ electron configuration but vanadium has a $ {d^3}{s^2} $ electron configuration.

Note:
In the case of Nb and V, we can conclude that the size of the orbital trend dominates over the energy gap in case of Niobium, because of this niobium has a $ {d^4}{s^1} $ electron. Unusual electronic configurations are very common in case of transition metal. Polonium is also another transition metal with exceptional electronic configuration.