Understanding orbital configuration

[Im Learning]

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Neutral chromium is [Ar]4s1 3d5

I don't understand how their can only be one electron in the 4s orbital.

Does 4s fill after 3d or before?

Will it lose 4s electrons before or after 3d electrons?

 

[whiteshadodw]

Neutral chromium is [Ar]4s1 3d5

I don't understand how their can only be one electron in the 4s orbital.

Does 4s fill after 3d or before?

Will it lose 4s electrons before or after 3d electrons?

half-filled stability of the d-orbital allows the 4s orbital to be half filled as well. all the elements in that column have ns1 (n-1)d5 electron configuration. if chromium was to become oxidized, then it would lose its electron from the 4s orbital yielding Cr +1 1s2 ... 3s2 3p6 3d5