The electronic configuration of Cr is (atomic number = 24):
(a)- $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2} 4 p^{4}$
(b)- $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{6}$
(c)- $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{5} 4 s^{1}$
(d)- $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{1} 4 p^{5}$

Question

The electronic configuration of Cr is (atomic number = 24):
(a)- $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2} 4 p^{4}$
(b)- $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{6}$
(c)- $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 3 d^{5} 4 s^{1}$
(d)- $1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{1} 4 p^{5}$

Vedantu Content Team · Accepted Answer

Hint: Cr is the chromium element that belongs to the d-block. The last electron enters into the d-subshell. The d-orbital has the property of extra-stability because of a half-filled or fully-filled subshell.Complete step by step answer:Cr is the chromium element and it belongs to transition elements and is the element of d-block.The atomic number of chromium is 24 and it belongs to the first series of the d-block.According to the Aufbau principle the order of filling the electron in the orbital according to their increasing energies is: $\text{1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s}$ So, according to this order of filling the configuration of chromium must be:$1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{2}}3{{d}^{4}}$.But this is not the correct configuration of chromium. The correct configuration of chromium can be explained by:There are some exceptions in the first series in the electronic configuration of chromium (atomic no. = 24). It must be noted that unlike other elements of the first transition series, the chromium has 1 electron and is $4s-orbital$. This is due to the gain of additional stability by the atom by having a half-filled configuration (because the d-orbital has 5 subshells and each subshell has 1 electron) of $3d-orbital$. The $3d-orbital$in case of chromium gets exactly half-filled with configuration of $3{{d}^{5}}4{{s}^{1}}$. So, the configuration of chromium is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}3{{d}^{5}}4{{s}^{1}}$.Hence, the correct answer is an option (c)- $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}3{{d}^{5}}4{{s}^{1}}$.   Note: In the first series of the transition metals the copper atom has also an exception in its configuration. The configuration of copper is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}3{{d}^{10}}4{{s}^{1}}$. This is because the d-subshell has complete 10 electrons that have extra stability.

The electronic configuration of Cr is (atomic number = 24):(a)- 1s22s22p63s23p64s24p4 (b)- 1s22s22p63s23p63d6 (c)- 1s22s22p63s23p63d54s1 (d)- 1s22s22p63s23p64s14p5

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