Hi Dr. Sharma,

This is Siddharth, your student in AP Chemistry on Saturdays and Sundays. I was studying for my unit 1 exam and found a lot of exceptions to the Aufbau principle. I know that some of the elements want half filled and fully filled shells, so they shift electrons a bit, but there are some weird electron configurations like:

Ruthenium:
Expected = [Kr] 5s2 4d6
Actual = [Kr] 5s1 4d7

which had a fully filled s subshell before moving one electron to make a half filled s subshell and a somewhat filled d subshell. Why is this? Also, there are some more weird happenings in the f block. Some periodic tables are saying the electron configuration of Cerium is

[Xe] 6s2 5d1 4f1, whereas some are saying it is [Xe] 6s2 4f2. Which one is right? Is there a difference? Again, there are more elements that are being strange, such as Pr (Praseodymium), which has an electron configuration (according to the web) as:

[Xe] 6s2 4f3. Is there a reason for this? Why is it 4f3, when it is only the second element in the f block? Is it because lanthanum is considered part of the f-block?

Also, last question, are there any more notable exceptions to the Aufbau in the d and f blocks? How do we know the valence electrons in the d-block? Some sources are saying include s and d subshells even if d is a full number lower, while some others are saying only include s, so most of the d block elements should have only 2 valence electrons.

Hi Siddharth,

It is true that there are many irregularities in which Aufbau Principle is violated in d and f block elements.
1)    One violation is reasonable which as you have mentioned also due to exactly half filled or fully filled subshells.
2)    The other irregularities observed are because 6s, 5d and 4f orbitals are of nearly similar energies in lanthanum series. Due to this element acquire some special electron configurations to minimize the inter electron repulsions as electrons can exchange their positions easily when adjacent orbitals have similar energies.
Ruthenium:
Expected = [Kr] 5s² 4d⁶
Actual = [Kr] 5s₁ 4d⁷
Similar complexities occur among the elements in actinium series, in which the 7s, 6d, and 5f orbitals have close energies.
3)    Lanthanum has a configuration of [Xe]6s²5d¹, and it is considered as d block element. However, the next element, cerium has the configuration of [Xe]6s²4f¹5d¹ and is f block element.

4)    Check electron configuration of gadolinium is [Xe]6s²5d¹4f⁷, rather than
the predicted [Xe]6s²4f⁸
5)    Pr (Praseodymium)- [Xe] 6s² 4f³ rather then [Xe]6s²5d¹4f² again because of very close energies of 6s,4f and 5d.
6)    Valence electrons are generally s and p electrons in the main periodic table. However, in d and f block elements, electrons of s subshell are lost first followed by d and f subshell electrons. Generally, all d block elements show +2 oxidation number and higher oxidation numbers are achieved from inner d or f subshell electrons removal. So we can say valence electrons in these elements are from s, d and f orbitals.
7)    Here are the configurations of all f and d block elements. (You need not to know these exceptional configurations from 4d ,5d and 6d for AP chem exam)
I have boxed all exceptional electron configurations.