Definition of the Aufbau principle

The Aufbau principle determines, through certain rules, how electrons are organized in shells and subshells around the atomic nucleus:

• Electrons enter the subshell that has the lowest possible energy.
• An orbital can contain a maximum of two electrons, obeying the Pauli exclusion principle.
• Electrons follow Hund’s rule. It states that electrons are scattered if there are two or more orbitals that are equivalent in energy.
• The lower electron orbitals are filled before the higher orbitals forming the electron shell. The end result is that the atom, ion, or molecule forms the most stable electronic configuration possible.

The Aufbau principle covers energy levels 1 to 7, and the sublevels: “s” (2 electrons), “p” (up to 6 electrons), “d” (up to 10 electrons) and “f” (up to 14 electrons). ).

In the elementary state of an atom, electrons occupy atomic orbitals. The Aufbau construction method is the procedure that establishes the electronic configuration of an atom and consists of following an order to fill the different orbitals, based on the energy values ​​of each one of them. For this, the Möller diagram or the diagonals are used, as well as the minimum energy rule (n+l): “n” is the principal quantum number and “l” the azimuthal quantum number.

Examples of the Aufbau principle

Through these rules, the Aufbau principle provides us with a series of instructions for the correct location of the electrons in the orbitals of an atom. Some examples to observe the electronic configuration according to the Aufbau principle are:

• Hydrogen (Z=1): 1s ¹
• Carbon (Z=6): 1s ²  2s ²  2p ²
• Boron (Z=5): 1s ²  2s ²  2p ¹
• Nitrogen (Z=7): 1s ²  2s ²  2p ³
• Oxygen (Z=8): 1s ²  2s ²  2p ⁴
• Potassium (Z=19): 1s ²  2s ²  2p ⁶  3s ²  3p ⁶  4s ¹
• Iron (Z=26): 1s ²  2s ²  2p ⁶  3s ²  3p ⁶  4s ²  3d ⁶
• ^{Tin (} Z=50): 1s ²  2s ²  2p ^{6 3s 2}  3p ⁶  4s ²  3d ¹⁰   4p ⁶ 5s ²  4d  10  5p ²

Limitations of the Aufbau principle

The Aufbau principle helps to predict the configurations of most elements. However, as with most rules, there are also exceptions to the Aufbau principle: some elements do not always follow it. For example, the predicted electron configuration for chromium (Cr) is 4s ² 3d ⁴ , but the observed configuration is actually 4s ¹ 3d ⁵ .

Another example is copper (Cu). Its electron configuration according to the Aufbau principle is 1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ 4s ² 3d ⁹ , but it was proven experimentally that it is actually 1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ 4s ¹ 3d ¹⁰ .

Bibliography

• Pharmacy faculty. The electronic structure of atoms. (1993). University of Alcala. Available at: http://www.geocities.ws/ingenierosistemas1/estructuraelectronicaatomos.pdf
• Grillo, C. Aufbau Principle or Rule of the Saw. (2013). Available at: https://youtu.be/EiT0qHAKeRs
• Blanco Ramos, F. Introduction to the Physics of Atoms and Molecules . (2019). Spain.