Question

What is the electronic configuration of atomic chromium, $Z=24$ ?

Answer 1

The electronic configuration of an element is written in the ascending order of their shells filled. The electronic configuration of an element is also written as the name of the noble gas which has lesser orbits filled and then the orbits filled afterwards.

Complete answer:
The electronic configuration of chromium, which has the atomic number 24, is $[Ar]3{{d}^{5}}4{{s}^{1}}$ .
Let us first know how an electronic configuration is written. To write the electronic configuration of an element, we first have to look at its atomic number. Because, the atomic number represents the amount of protons in the nucleus of its atom.
And, the amount of protons and electrons are the same in an atom of any element. Thus, by looking at the atomic number of that element we identify the total number of electrons it has in its atom.
Now, we start to fill the orbitals one by one, first s orbitals are filled, then p orbitals, then do orbitals and then f orbitals. So, here in chromium we have 24 electrons, first we fill the 1s orbital by putting two electrons into it.
So now we have 22 electrons left, we put two electrons in 2s orbitals, now we have 20 electrons left. Now, 2p orbital is filled, and so we put 6 electrons in the 2p orbital. Thus, we remain with 14 electrons and then we put another two electrons in the 3s orbital.
After the filling of 3s orbital we fill the 3p orbital and thus we put another 6 electrons in the 3p orbital. Now, we are left with 6 electrons. Now, we fill the 4s orbital with another two electrons.
After 4s is filled we put the remaining 4 electrons in the 3d orbital. Thus the electronic configuration of chromium becomes $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}4{{s}^{2}}3{{d}^{4}}$ . But, due to an exception the aufbau’s principle does not work here and also half-filled and fully filled subshells have more stability. So, the electronic configuration of chromium is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}3{{d}^{5}}4{{s}^{1}}$ .
As the electronic configuration of Argon ( $Ar$ ) is $1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}$ , thus the configuration of chromium is written as $[Ar]3{{d}^{5}}4{{s}^{1}}$ .

Note:
The aufbau’s principle states that the orbitals of atoms of every element are filled according to a specified arrangement, but this arrangement is not applicable to many of the elements beyond the atomic number of 20.