It might be that all this reasoning about shells want to be half full and so on is correct, but there are counter examples. Keeping in mind what has been said in this thread, you should even be able to figure out the 'odd' ones. This is nice and stable and everybody's happening.įor fun, pick any random d or f block element and write out it's electron configuration before you look it up. As a result, you get a completely filled 3d shell and a half-filled 4s shell. So in copper, an electron is taken from the 4s shell and added to the 3d shell along with the extra electron that copper has over nickel. Anything in between isn't good and if it's possible to rearrange the electrons to accomodate that then you'll probably see it happen. Shells want to be either half full, or totally full. Since the 3d shell can hold a total of ten electrons, it would wind up being 9/10'th full which is an odd fraction. If you just add one electron to that as you move to copper, then you'd have four totally filled shells and one half-filled shell. In Nickel (4s2-3d8) you have four totally filled 3d subshells and one empty one. When you move from Nickel to Copper, you would logically think that you'd go from 4s2-3d8 to 4s2-3d9. There is no hard and fast rule for this, but that is an explanation that correlates with experimental data.Remember that the 4s shell fills in before the 3d shell does, because as Yggdrasil pointed out, their energy levels are pretty much the same. Thus, electron pairing is favorable enough for Tungsten. The more the electron distribution is spread out, the less electron-pair repulsion there is, and thus the lower #Pi_c# is. However, Tungsten's #5d# and #6s# orbitals being larger than the #3d# and #4s# orbitals (respectively) spreads out the electron density enough that the pairing energy ( #Pi = Pi_c + Pi_e#) is small enough. The small-enough orbital size means that the electron density is not as spread out as it could be, which makes it favorable enough for a maximum total spin to give the most stable configuration.The minimization comes from having all unpaired electrons in the #3d# and #4s# ( #3d^5 4s^1#), rather than one electron pair in the #4s# ( #3d^4 4s^2#). The minimized coulombic repulsion energy #Pi_c# further stabilizes this configuration. ![]() The maximization comes from how there are #5# unpaired electrons, instead of just #4# ( #3d^4 4s^2#). The maximized exchange energy #Pi_e# stabilizes this configuration ( #3d^5 4s^1#).One explanation for Chromium, then, is that: The former is stabilizing and the latter is destabilizing, as shown below (suppose configuration 2 is at pairing energy #Pi = 0#): These combine to produce an overall pairing energy #Pi = Pi_c + Pi_e#.The coulombic repulsion energy #Pi_c# (a destabilizing factor that is inversely proportional to the number of electron pairs).The exchange energy #Pi_e# (a stabilizing quantum mechanical factor that is directly proportional to the number of pairs of electrons in the same subshell or very close-energy subshells with parallel spins). ![]() ![]() To explain Chromium's electron configuration, we could introduce: Unfortunately, there is no easy way to explain these deviations in the ideal order for each element. Interestingly enough, Tungsten is more stable with an electron arrangement of #4f^14 5d^4 6s^2#.
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