Question

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

Answer 1

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:
$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^4$.
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 $3d^{5}4s^1$. So, the configuration of chromium is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^1$.

Hence, the correct answer is an option (c)- $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{5}4s^1$.


Note: In the first series of the transition metals the copper atom has also an exception in its configuration. The configuration of copper is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^{10}4s^1$. This is because the d-subshell has complete 10 electrons that have extra stability.