Question

Identify the correct ground state electron configuration for Cr.
(A) $$[Ar]3s^{2}3d^1$$
(B) $$[Ar]3s^{2}3d^5$$
(C) $$[Ar]4s^{2}3d^5$$
(D) $$[Ar]4s^{1}3d^5$$

Answer 1

Hint: Atomic number of Chromium is 24. Arrange the electrons in the orbitals in a way that Hund’s rule and Aufbau’s rule is followed. There can be exceptions also in those rules if more stable electronic configuration is possible.


Complete step by step solution:
- Cr is a chemical symbol of element Chromium. Atomic number of Cr is 24. So, it will have 24 electrons in its ground state. Let’s organise it the orbitals according to the rules.
- So, its ground state configuration can be $$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^4$$ which can be written as $$[Ar]4s^{2}3d^4$$ because noble gas [Ar] has electronic configuration $$1s^{2}2s^{2}2p^{6}3s^{2}3p^6$$.

- So, if we follow Hund’s and Aufbau principles, electronic configuration of Cr should be $$[Ar]4s^{2}3d^4$$ but it has an exception in electronic configuration of elements.
- We know that d-sublevel has 5 orbitals and if 5 orbitals are filled by 5 or 10 electrons, then they have exceptional stability. Now , if one electron of 4s orbital gets excited to 3d orbital, then 3d orbital will get its stable state and that’s why this electronic configuration is the ground state of chromium element because it has higher stability than other configurations.
- So, ground state electronic configuration of Cr is $$[Ar]4s^{1}3d^5$$.

Additional Information:
- Remember that apart from Chromium, Copper and Molybdenum’s electronic configuration are also exceptions.
Copper’s electronic configuration: $$[Ar]4s^{1}3d^{10}$$
Molybdenum’s electronic configuration: $$[Kr]4d^{5}5s^1$$


Note: Chromium's electronic configuration is an exception and it has different configuration as suggested by Hund and Aufbau’s principles. Do not just write the electronic configuration of any species by following just Hund’s and Aufbau’s rules, there can be exceptions if more stable electronic configuration is possible.