|MadSci Network: Chemistry|
there is one answer you can use in (nearly) every case in chemistry: "The energy is lower." - and this is also true in this case. In chemistry there is a principle called the "Aufbau principle", where Aufbau is German and means "structure" or "setup" - so it is the principle which governs the electronic occupation of the orbitals. But the cases you are talking about are the EXCEPTIONS from this principle. You can find those exceptions if half or completely filled sub-shells can be created if you deviate from that principle. What does that mean?
Within one shell (1,2,3 ...) you have sub-shells, call them "orbitals", if you want to: 3dz for example is an orbital which belongs to the 3d sub-shell. You have five d-orbitals so you can accomodate 10 electrons. If every of these orbitals is filled with one elctron, it is half-filled and is quite stable, even more so if it is competely filled. You can say that it is quite simple to say "This occupation has a lower energy!", but you can also measure it by IR spectra to prove it is right. If you are interested, have a look at http://amug.o rg/~rwiley/chapter_twenty_two-transit.htm and look for "Exceptions to the AUFBAU Filling Principle" in the middle of the page.
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