Aufbau Principle/3d-4s Configurations

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Alex Nguyen 3I
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Aufbau Principle/3d-4s Configurations

Postby Alex Nguyen 3I » Mon Oct 19, 2015 3:06 pm

Can someone explain again the reasoning behind 4s being a higher energy state only when the 3d state is being or has been filled?

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Re: Aufbau Principle/3d-4s Configurations

Postby Chem_Mod » Tue Oct 20, 2015 12:10 pm

Starting at scandium (Sc) the 3d orbitals are lower in energy than the 4s. This is what one would expect:
Lower energy 3s < 3p < 3d < 4s Higher Energy

The question is why is scandium's ground-state electron-configuration [Ar]3d14s2 and not [Ar]3d3?

The energy difference between the 3d and 4s is small which means electron-electron repulsions will play a role.
For scandium a second electron in the 3d state must result in higher electron-electron repulsion and therefore the 2 electrons are in the slightly higher energy 4s state resulting in the experimentally-observed ground-state [Ar]3d14s2.

As stated in class many times the only electronic configurations we discuss or use are the ground state written from lowest energy to highest energy as listed in your textbook and NIST.

See textbook and reference tables and data at the National Institute of Standards and Technology (NIST):
http://physics.nist.gov/PhysRefData/DFT ... ation.html

Both are consistent with experimental observation (spectroscopic data) and calculations.

As discussed in class writing the ground-state electron-configuration for ionized atoms (cations) is straight forward as the highest energy (easiest to remove) electron is omitted from the neutral-atom ground-state electron-configuration.

For example Sc+: [Ar]3d14s1. Which is the same as the experimentally observed ground-state for Sc+ as listed at the above NIST link.

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Re: Aufbau Principle/3d-4s Configurations

Postby Chem_Mod » Tue Oct 20, 2015 2:59 pm



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