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# The electronic configuration of chromium in its ground state is

$\begin{array}{1 1}(a)\;[Ar]4d^44s^r&(b)\;[Ar]4d^54s^1\\(c)\;[Ar]3d^43s^r&(d)\;[Ar]3d^54s^1\end{array}$

Answer: The electronic configuration of chromium in its ground state is $[Ar]3d^54s^1$
Cr is an example of an exception to the Aufbau principle. If there are 4 electrons, or 9 electrons in the d orbital, it will move one electron from the s orbital below it to fill the extra space.
Cr's electron configuration, following the model would be: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^23d^4$, but instead it is $1s^2 2s^2 2p^6 3s^2 3p^6 4s^13d^5$, i.e, $[Ar] 3d^54s^1$, because there is extra stability gained from the half-filled d orbital.
edited Aug 5, 2014