MadSci Network: Chemistry |
Hi Ed,
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.
Bye,
Andreas
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