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# The Aufbau Principle - Electronic Structure and the Aufbau Principle

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The Aufbau Principle - Introduction to the Aufbau Principle

Graph showing energy of electron orbitals by quantum energy number.

Todd Helmenstine
Stable atoms have as many electrons as they do protons in the nucleus. The electrons gather around the nucleus in quantum orbitals following four basic rules called the aufbau principle.

• no two electrons in the atom will share the same four quantum numbers n, l, m, and s.
• electrons will first occupy orbitals of the lowest energy level.
• electrons will fill an orbital with the same spin number until the orbital is filled before it will begin to fill of the opposite spin number.
• electrons will fill orbitals by the sum of the quantum numbers n and l. Orbitals with equal values of (n+l) will fill with the lower n values first.
The second and fourth rules are basically the same. The graphic shows the relative energy levels of the different orbitals. An example of rule four would be the 2p and 3s orbitals. A 2p orbital is n=2 and l=2 and a 3s orbital is n=3 and l=1. (n+l) = 4 in both cases, but the 2p orbital has the lower energy or lower n value and will get filled before the 3s shell.
##### Further Reading on Electron Configurations

Anne Marie Helmenstine, Ph.D.