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)- $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{1}}4{{p}^{5}}$

Answer 1

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.