Why and How Does the f-block elements differ in e.config. from aufbau principle

[Maisara-WD]

Hi all

I cannot understand

Why Does the f-block elements differ in their electronic configuration from aufbau principle... Please I want to understand this particular mystery..

thanx

 

[Borek]

Please give examples of what you mean.

 

[Maisara-WD]

That's aufbau sequence:

1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p

see the e. configuration of cerium (58Ce) foe instance:

[Xe] 6s2, 4f1, 5d1

unlike that stated in the aufbau principle... why isn't the 4f filled completely before 5d??

 

[Maisara-WD]

Similarly and more obviously...

57La.. Lanthanam...

[Xe] 6s2,"5d1"

This is the problem.. why didn't we enter the 4f sublevel directly..?! this outermost configuration led to putting the La in the d-block elements and not f-block..
and all lanthenides are following the general configuration: [Xe] 6s2,5d1,4f(1-14)
those are the 14 elements of the lanthenides...

 

[DrDu]

The aufbau principle is not a fundamental law of physics. It only holds for atoms for which a self consistent field description of the one-electron states is sufficiently accurate and leads to a central potential which differs not too much from the form Z_eff/r. However, in actual atoms, there is a continuous transition from a Z/r like behaviour very near the nucleus, where there are no other electrons to shield the nuclear charge Z, and 1/r at large distances, where all other Z-1 electrons shield the nuclear charge Z. This r-dependent shielding of nuclear charge can be roughly described by a potential of the form Z_eff/r+a/r^2. The additional term has the effect to split the levels of different angular momentum which for a pure Z_eff/r potential would be degenerate. Hence ultimately the orbitals of high angular momentum and quantum number n overtake the states of low angular momentum with radial quantum number n+1.

 

[Borek]

That's aufbau sequence:

1s<2s<2p<3s<3p<4s<3d<4p<5s<4d<5p<6s<4f<5d<6p<7s<5f<6d<7p

Yes and no.

Sequence that you have listed is just a simplified version of the more general definition. Aufbau principle states that electrons fill orbitals starting at the lowest available energy. In the cases you have listed energies are going up the way they should, so Aufbau principle holds, just the sequence is a little bit different from the simplified version.