The atomic number of Cr is 24. There are 6 electrons in the outer two shells of Cr that is, 3d and 4s.
The assumed electronic configuration of Cr is [Ar]3d44s2
Filling of electron take place in increasing order of n + l value of orbital.
For 3d orbital,
Substitute, 3 for n and 2 for l thus,
n + l = 3 + 2 = 5
Similarly, for 4s orbital substitute, 4 for n and 0 for l thus,
n + l = 4 + 0 = 4
Hence, 4s should be filled first as 4s2 and remaining 4 electrons goes 3d as 3d4. But while writing 4s2 is written after 3d4.
The actual electronic configuration of Cr is [Ar]3d54s1. This is because half or full-filled orbitals are more stable than partially filled.
The d subshell has five orbitals. This subshell is extremely stable when it is empty or half-filled. So, Cr exists as [Ar]3d54s1 and not as [Ar]3d44s2 to acquire half-filled configuration.
Removing 3 electrons from Cr, Electronic configuration of Cr+3 is [Ar]3d34s0..
Electronic configuration of Cr+3 is [Ar]3d34s0.
To form a trivalent cation of Cr atom, 3 electrons are removed from its outer shells. One electron of the 4s shell and two electrons from the 3d shells.