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.


1 Answer 1


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.

  • 2
    $\begingroup$ 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
    Commented Aug 1, 2019 at 23:52