Why isn't Br an exception when writing electron configurations? Shouldn't it be Br [Ar] 4s1 3d10 4p6 instead of Br[Ar] 4s2 3d10 4p5, since atoms favour filled or half-filled shells?

Question

rogue · Answer

Transition generally metals have half-filled shells, not nonmetals like Br :P
So it's the second one...$$[Ar] 4s^2 3d^{10} 4p^5$$

wondermath · Answer

oh haha thx!

rogue · Answer

lol, I typed "Transition generally metals"
It should be "transition metals generally"

jfraser · Answer

the reason chromium and copper can be exceptions is because of the very narrow energy gap between the 4s and the 3d's, so the energy cost to jump is made up by the stability of the filled sub-level.

Trying to go from 4s to 4p is too big a jump, so the energy benefit of having a filled level isn't offset by the energy it takes to jump that high.