Question

IN ATOMIC STRUCTURE

IN Section ELECTRONIC CONFIGURATION AND ORBITAL DIAGRAMS (video 22:17)

The teacher is telling that Aufbau and Hund's rule have some exceptional case

Due to the atoms with half or fulfilled orbitals have more stability

example: copper and chromium

cr: [Ar] 3d⁴ 4s² (expectation)

[Ar] 3d⁵ 4s¹ (real)

this due to the atoms with half or fulfilled orbitals have more stability

my doubt is about

consider carbon and fluorine

C: 1s² 2s²2p² (I studied)

when I apply the half or full filled subshell has more energy

I will get this configuration

C: 1s² 2s¹2p³ (my doubt)

here s and p subshell is half filled I asking you that this model much stable than that I studied

same for fluorine

Answer 1

carbon :
atomic number = 6
C: 1s² 2s²2p²

fluorine:
atomic number = 9
F : 1s² 2s²2p5

The electron configuration of an atom,molecule or ion is determined on the basis of the so-called Aufbau principle. According to this principle, electrons start off by filling the lowest available energy levels first before filling the higher energy levels. The order in which the atomic orbitals are filled is dictated by the n+l rule,where orbitals with lower n+l values would get filled first. In the case where two orbitals have the same n+l value,the one with lower n is filled first. From this,it's easy to see why the electron configuration of C is 1s² 2s²2p² . Hund's rules,on the other hand, are used for determining the term symbol for the ground state configuration of an atom. To infer that the Hund's rules imply half-filled and full-filled configurations as the most stable configuration(as often found in the elementary chemistry textbooks) is both erroneous and misleading.

In case of copper 4s orbital is outside 3d. thus it can remain not filled completely. But in case of carbon 2s lies inside of 2p. thus only when 2s is filled completely 2p can be filled.