Electron configuration of:
Cr = 1s2 2s2 2p6 3s2 3p6 3d5 4s1
Cr2+ = 1s2 2s2 2p6 3s2 3p6 3d4
Cu = 1s2 2s2 2p6 3s2 3p6 3d10 4s1
Cu2+ = 1s2 2s2 2p6 3s2 3p6 3d9
Co = 1s2 2s2 2p6 3s2 3p6 3d7 4s2
Co3+ = 1s2 2s2 2p6 3s2 3p6 3d6
s can max contain 2 electrons.
p can max contain 6 electrons.
d can max contain 10 electrons.
f can max contain 14 electrons.