The atomic number of Chromium is Z=24Z=24, therefore a CrCr atom possesses 2424 electrons.
Cr:1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(1)3d^(5)Cr:1s22s22p63s23p64s13d5
Note that it is 4s^(1)3d^(5)4s13d5 and not 4s^(2)3d^(4)4s23d4 because a half filled dd orbital is more stable than a partially filled dd orbital.
However, the chromium ion Cr^(3+)Cr3+ possesses 24e^(-)-3e^(-)=21e^(-)24e−−3e−=21e− due to the loss of 33 of its electrons.
Thus, the electron configuration of Cr^(3+)Cr3+is:
Cr^(3+):1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)4s^(color(red)(0))3d^(color(red)(3))Cr3+:1s22s22p63s23p64s03d3