Aufbau in 3d series

[Ankit Gupta]

According to aufbau principle 4s will fill before 3d orbital because it has lower energy ,OK ,but now I read that in scandium and other 3d elements ,when donating electrons 4s will go first and then 3d .So please explain me why this happens , didn't found any explanations in books I have ,and on Google I found that this is because higher principal number of 4s,but it looks like more of a definition than explanation.

[fiveworlds]

OK ,but now I read that in scandium and other 3d elements ,when donating electrons 4s will go first and then 3d

 

It isn't as simple as that according to Atkins Physical Chemistry whether atoms donate their electrons from 4s or 3d depends on the effective nuclear charge Z_eff

[Enthalpy]

One fundamental reason for irregular filling is that electrons repel an other, so the energy levels you observe by, say, having all 3d and 4s void and available for one single electron do not apply for further electrons, when some are already present as 3d or 4s.

 

An other, less simple reason is that wavefunctions for electrons must be antisymmetric, and this influences the orbital shape a lot. The mere presence of an other electron changes the shape of the other orbital - provided you want to consider orbitals as functions of one single location. Have a look at ortho and parahelium, which differ by the spin alignment of both electrons:

http://www.ipf.uni-stuttgart.de/lehre/online-skript/f40_03.html and

http://hyperphysics.phy-astr.gsu.edu/hbase/atomic/grotrian.html#c1

 

You observe that Cr and Cu put one electron more on 3d and one less on 4s.

(click to enlarge)

 

So why would Ca organize its electrons as [Ar]4s² while, with just one proton more, Sc⁺ would put them as [Ar]4s3d? It could possibly be that 3d is, as a mean, nearer to the nucleus than 4s, so one proton more favours 3d. I'm not quite convinced because s orbitals have a density at the nucleus while d have zero density there.

 

Less simple: chemists are interested in atoms and ions in crystals, solvents... rather than alone in vacuum, and 3d may be more favourable in the context.