h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I´m looking at the way orbitals fill up with electrons, and one thing has me puzzled. If you look at vanadium, it has 3 electrons in the 3d shell, and 2 electrons in the 4s shell. But then chromium which has an extra electron, for some reason one electron from 4s decides to jump down and spend time with the new one in 3d. So that chromium instead has 5 electrons in 3d and 1 in 4s. I can see it happens a few other places as well. Why doesn´t the electrons in 4s just stay there? So you would have 4 in 3d and 2 in 4s.

In short - certain electron configurations are more stable than others. Hence why elements with paired electrons are more stable than those with unpaired electrons and so forth. And elements will contort the simple rules of the Periodic Table a little bit in order to be more stable.

The d-shell has five orbitals, which will hold ten electrons. Each orbital has two electrons with opposite 'spins' (not actual spin - a quantum technicality, used to make the sums work). When each orbital is half-full with electrons of similar spin, this is a very stable state; more stable than a full s-shell. Conversely, having only four filled orbitals is very unstable, less so than the half-filled 4s shell.

In 'real world' terms, you could think of it as conflicting charges (the electrons) repelling each other. Some arrangements of charges just do not work. But, if you throw another charge into the mix, then the patterns change, and often the charges will take up the previously 'impossible' arrangement. This sort of idea can be replicated with magnets.

Interesting one, Xanatic, takes me back a few years - but _we_ didn't go quite so deep.

Mu Beta: I can 'see' that some configurations are more stable than others (without necessarily understanding the technicalities).
Now, before anyone else asks... Magnets - how?

Think of the little executive game with the swinging ball and magnets on the base. If you put four magnets on the base, then the magnetic fields will configure the ball to move in certain ways. Add a fifth magnet and the ball will move in different patterns. Take away a magnet and try to replicate the magnetic fields when you had five magnets - it can't be done. Hence you are creating distinct field shapes with the juxtaposition of charges - which, despite all the quantum theory, is simply what electronic orbitals are.

B

Right. That's good and clear and understandable.

OK, I think I've got the principals of the principle.

Thanks, Mu Beta.

Thanks Beta. I thought it would be due to some kind of specific rule, but I guess it is just an exception you have to remember.

Well, if you can remember that the d-shell is most stable when it contains either 5 or 10 electrons, that should see you straight.

B

Oh, by the way, it's usually at about this point that BigAl or Orcus turn up and tell everyone that I'm talking complete bollocks.

B

Hang about! When did that word come off the verboten list? Now my 'loblocks (anag.)' gag doesn't work any more

It would appear that the profanity filter has been lifted altogether.

Boobs, cock, piss.

B

I'm pretty certain you could say all those words anyway. If the filter has been lifted then I should be able to write a certain word, which I'm not going to try to do, without my post bouncing back.

Think you're OK this time B

With the science or the childish swearing?

B

Naturally.

Actually I didn't think I had much to add to this that didn't involve terms such as 'exchange integrals' which would be fairly meaningless to the majority but it does occur to me that it's not just elemental chromium that does this. The ions of various transition metals such as iron 2+ also have forms where they exist in the so-called high-spin form.
Carbon doesn't bond to most other elements in its lowest energy form it promotes an electron from its 2s orbital into the 2p so as to have the configuration 2s12p3 in its outer shell and so pairs up each of these in bonds to form all its 4-coordinate compounds.
Also, molecular oxygen is in a high-spin form (two unpaired electrons in its highest occupied molecular orbital).

So this is not a property unique to elemental chromium, it crops up quite a bit in fact.

It is a good explanation, and I also felt that it wouldn't help to add that elemental Cr and V are solids, and they actually have energy bands rather than orbitals, with various degrees of s & d mixing depending upon the electron wavevector.

'Oh, by the way, it's usually at about this point that BigAl or Orcus turn up and tell everyone that I'm talking complete bollocks'

Yeah I've been ill with tonsillitis for a full fortnight and haven't been in the mood for doing ANYTHING. I've never had such a debilitating condition in all my life!

Anyway, what MB said in Post 2 is essentially correct. The only thing I would add is that energy of the 3d sub-shell is only marginally higher than the 4s sub-shell. So when the element chromium is being built up, because it only needs 1 more electron to form the more stable 'half-full d-shell', one of the 4s electrons is very easily promoted to give 3d5