Hi Iam having a little problem understaning why some atoms are configured in a certain way. I know that each sub-shell can take 2(2l+1) electrons, so for s->2 p->6 d->10 But why e.g does the atom, Cr (Z=24) have the configuration : ...3d54s1 rather than ...3d6 ? I know about that electrons in d sub-shell has higher energy than s and p, ( s<p<d ) Is this something we just have to accept because we know from experiments that Cr has the total energy X, so it cant possibly have ...3d6? Or is there some other explanation? Thanks!