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It has something to do with the stability of the element and how it likes a specific configuration. The sublevels when they are completely filled are more stable than those that are partially filled. Sorry this is a kind of convoluted explanation but it is the only one I have. I hope more people can weigh in on this.
The exceptions are slightly different for every element, but some of them revolve around prioritizing stability in the sub shells by either filling them up halfway or taking an electron from another sub shell to completely fill a sub shell. This is the case with Copper; to get a more symmetric distribution that will lead to more stability in the atom, an electron from the 4s shell in copper will move to the 3d shell so that there is a completely filled 3d orbital (why we have the 4s1d10 electron configuration). A similar rule occurs in chromium, but the shell will instead fill up 3d shell halfway (4s1d5) to maximize stability. There are more exceptions the farther up you go on the periodic table (mainly because of how small the differences in energy are between the sub shells at higher energy levels), but most of those exceptions are outside of the scope of Chem 14A.
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