Question

Certain elements do not follow aufbau principle. Enlist atleast four. Indicate the configuration following aufbau and mark the deviations. Clearly explain why these elements do not follow the principle at that specific level for each of the four elements

Answer 1

The four elements which don't follow aufbau principle are: Cr, Cu, Pd, La.

According to the aufbau principle:

1. Cr=[Ar]3d⁴4s² but experimentally it is [Ar]3d⁵4s¹ , Ar is argon.

This is because d⁵ is half filled shell which is much more stable than the fully filled 4s² So d orbital takes one electron of 4s orbital and become more stable.

2. Cu=[Ar]3d⁹4s² but experimentally it is [Ar]3d¹⁰4s¹

This is because 3d is much more stable with the fully filled orbital i.e. 10 electron rather than having one less electron.

3. Pd=[Kr]4d⁸5s² but experimentally it is [Kr]4d¹⁰5s⁰ , Kr is krypton

This is because Full d orbitals are more stable than partially filled ones. And at higher energy levels,the levels are said to be degenerated which means that they have very close energies and then electrons can jump from one orbital to another very easily.

4. La =[Xe]6s²4f¹ but experimentally it is [Xe]5d¹6s²

where, Xe is the noble gas Xenon .But it seems this is one of the exceptions to the filling order of orbitals. Some elements are more stable with less electrons in some sub-shells and more electrons in others . The 5d and 4f sub-shells have very close energies. Lanthanides has a special feature known as Lanthanide contraction. This Lanthanide contraction explains why 5s and 5p orbitals Penetrate the 4f sub-shell and filling of electron takes place in 5d sub shells instead of in 4f subshell.