Electron Configurations of d-orbitals

[oxelhs]

Having a bit of trouble with the d-orbital electron configurations. For example, neutral Cr.

Why is the neutral configuration for Cr 4s13d5? Why isn't it 4s23d4?

Is it simply because d-orbitals fill first? And if this is the case, how come Co isn't just 3d9 without any s-obitals being filled?

Any help would be appreciated!

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[AureatEagle]

Having a bit of trouble with the d-orbital electron configurations. For example, neutral Cr.

Why is the neutral configuration for Cr 4s13d5? Why isn't it 4s23d4?

Is it simply because d-orbitals fill first? And if this is the case, how come Co isn't just 3d9 without any s-obitals being filled?

Any help would be appreciated!

It turns out that completely half-filled d shells (d5) and completely filled d shells (d10) are particularly stable configurations, similar in idea to the stability of an octet. As a result, very often if it is possible for a transition metal to promote a single 4s electron into the d shell and obtain one of these configurations, it will do just that. For example, the "expected" configuration of copper without rearrangement would be 4s2,3d9, but since the d-shell is just the one electron away from obtaining 3d10, it promotes a 4s electron to do the d-shell such that it becomes 4s1, 3d10.

The 4s shell definitely fills first.

 

[oxelhs]

Ahh okay. So Cu and Cr are really the only exceptions correct?

 

[AureatEagle]

Ahh okay. So Cu and Cr are really the only exceptions correct?

No, you'd also see the same principle applying to Mo, Ag, Au (analogous atoms in d4 and d9 columns). Any further down and it gets complicated, though.

 

[oxelhs]

Ah yes, sorry I meant in the 3-d metals. Thanks for clearing it up!