# Why 4s lose electron to 3d? [duplicate]

In chromium and copper, the are the configuration fill up like $$\mathrm{3d^5 4s^1}$$ but not $\mathrm{4s}$ first as it has lower energy level. and why only these 2 elements.

• Copper doesn't have $3d^5$ in its base state, but a completely filled $d$ orbital. – M.A.R. Jul 26 '15 at 16:16
• If you've just only been recently introduced to the electron configurations, then an appropriate answer would be: "the half-filled 3d orbital in Cr and the completely filled 3d orbital in Cu are particularly stable". For a more detailed but also potentially very technical answer, look up exchange energy. – orthocresol Jul 26 '15 at 16:23
• This is a very popular question in the chemistry world, and it has been asked and answered before: chemistry.stackexchange.com/questions/151/… – Ben Norris Jul 27 '15 at 1:06