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This question already has an answer here:

Orbitals fill in the following order, 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, etc. Its a nice pattern that many elements follow, but there are exceptions to. For example:

V: 1s2 ... 4s2 3d3
Cr: 1s2 ... 4s1 3d5

And not: (which would seem to be breaking pattern)

Cr: 1s2 ... 4s2 3d4

This can also be seen on other elements, where a higher shell 'steals' from one below it in a seemingly random order. Is there a reason behind this, and if so, even if complex, what pattern is there to it?

I am new to the world of the very small, new to chemistry. Any help is greatly appreciated.

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marked as duplicate by Qmechanic Aug 2 at 1:53

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The order you show applies to the H-atom. But electrons in larger atoms interact with each other as well as with the atomic nucleus. This causes some of the order you cited to break down.

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    $\begingroup$ Indeed, and it is perhaps to be marveled at that the one-electron levels for H pretty much keep their identities in 40- or 50-electron (and many more!) atoms. $\endgroup$ – Jon Custer Aug 1 at 23:52

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