Source: cirosantilli/exception-to-the-madelung-energy-ordering-rule
= Exception to the Madelung energy ordering rule
= Exceptions to the Madelung energy ordering rule
{synonym}
The most notable exception is the borrowing of 3<d-orbital> electrons to 4<s-orbital>[s] as in <chromium>, leading to a 3d5 4s1 configuration instead of the 3d4 4s2 we would have with the rule. TODO how is that observed observed experimentally?