[harini_5]

Hi everybody! according to afbau principle after the 6s orbital is filled 4f fills up and only then 5d gets filled. But this doesn't happen so. In barium the 6s orbital is complete and this according to afbau principle means that electron has to enter 4f. but this is not so.the element next to Ba is La where the last electron enters 5d and not 4f. is this in violation to afbau principle? I have learnt that the same happens in Ac also. Why does this violation occur?

[Alpha-Omega]

Barium and Lanthanum:

Ba:  Ground state electron configuration:  [Xe].6s²

La:  Ground state electron configuration:  [Xe].5d¹.6s²

The filling of electron shells depends on a number of factors.  The ground state electron configuration any element adopts will depend on energy an symmetry factors/considerations.

For example:

Chemically,  potassium behaves like sodium.  Both are alkali metals.  It appears the next electron is in an s orbital, not a 'd' orbital. For potassium, the energy of the 4s orbital is very close to the energy of the 3d orbital. However,  the energy of the 4s orbital is lower in energy compared to the 3d. So the next electron is placed into the 4s orbital.

Lanthanum:

At gallium we begin filling the 4p sublevel and continue to krypton. Rubidium fills the 5s, yttrium the 4d and indium the 5p. Cesium fills the 6s and lanthanum begins the first available f sublevel, the 4f. The f sublevel is filled from lanthanum through ytterbium. Throughout this period there are strange reversals of configurations.

See this link:  http://acswebcontent.acs.org/periodic/tools/PT.html

Click Electron Configuration Tab at the top of the page. At the bottom of ths page find Lanthanum.  Click on Lanthanum.  You will see the electron configuration.

See This Link:  http://lectureonline.cl.msu.edu/~mmp/period/electron.htm

From this Applet you can build electron configurations.

Arrangements of electrons in the orbitals of an atom is called its electron configuration. And we use the Pauli exclusion principle and other rules to arrange the electrons. To designate the electron configuration we use the level number and the letter of the sublevel and a superscript number to represent the number of electrons contained in the sublevel. Writing the electron configuration requires that we recall how many orbitals are contained in each type of sublevel.


The Aufbau Principle

The physical and chemical properties of elements is determined by the atomic structure. The atomic structure is, in turn, determined by the electrons and which shells, subshells and orbitals they reside in. The rules af placing electrons within shells is known as the Aufbau principle. These rules are:


 1. Electrons are placed in the lowest energetically available subshell.
 2. An orbital can hold at most 2 electrons.
 3. If two or more energetically equivalent orbitals are available (e.g., p, d etc.) then electrons should be spread out before they are paired up (Hund's rule).
 
I do not see that the Aufbau Principle is violated in the filling of Lanthanum. It is NOT THE ONLY PRINCIPLE observed for the filling.

Each orbital can only hold 2 electrons spinning in opposite directions as shown by the up and down arrows in the applets.  This means that each sublevel can hold 2, 6, 10, 14 electrons in the s, p, d, and f sublevels respectively.
Rule number ONE in all of chemistry is GAIN STABILITY.  All chemical reactions occur in order to gain stability.  In terms of the electrons they are most stable in the ground state or lowest possible energy. This means that we fill the lowest energy levels first.

However, as the number of energy levels and sublevels increases it becomes complex enough so that the energy levels overlap.

Example of extended periodic table:  http://jeries.rihani.com/index3.html

This periodic table below is a representation of how the d block is split by the f elements.

Remember that scandium (Sc), yttrium (Y), lanthanum (La), and actinium (Ac) are all part of the d-block in the
above table, even though they are separated from the rest of the d-block by the f-block.