Question

If the electronic configuration of calcium is [Ar]4s2 then what will be the electronic configuration of chromium and free dipositive manganese ion?

Answer 1

electronic configuration of chromium = [Ar]4s13d5
electronic configuration of dipositive manganese ion = [Ar]3d5

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Answer 2

The electronic configuration of calcium is $Ar[4s^2]$

Explanation:

Calcium has 20 electrons in total and 2 valence electrons. Electrons do not exist in the 3d orbital of calcium. In the periodic table, after calcium, electrons start filling the 3d orbitals which have less energy. After calcium in the periodic table, the 3d sub level falls below the 4s level.  

The electronic configuration of chromium is thus $Ar[3d^54s^1]$

Now basically, the 4s orbital must have 2 electrons but since the 3d orbital is half filled and less stable, so 1 electron from the 4s orbital shifts to 3d orbital making it more stable.

Manganese has the electronic configuration $Ar[4s^23d^5]$

The number of electrons in manganese in 25 and to achieve stability, the electron must be removed from 4s orbital.