Aufbau principle

Aufbau Principle

The Aufbau Principle is a fundamental concept in quantum mechanics that describes the process by which electrons fill atomic orbitals. The term "Aufbau" is derived from the German word for "building up" or "construction", reflecting the principle's role in explaining the sequential filling of orbitals in atoms.

Overview

The Aufbau Principle is based on the Pauli Exclusion Principle, which states that no two electrons in an atom can have the same four quantum numbers. This principle is used to determine the electron configuration of atoms, the distribution of electrons among available orbitals.

Principle

According to the Aufbau Principle, electrons occupy the lowest energy orbital available first. This is often referred to as the "bus seat rule" or the "energy ladder rule". Electrons will fill up the lowest energy level before moving to the next level. For example, the 1s orbital must be filled before the 2s orbital begins to fill.

Exceptions

There are exceptions to the Aufbau Principle, particularly when dealing with larger atoms. These exceptions occur due to the complexities of electron interactions and the relative energies of the orbitals. For example, in the case of chromium ([Cr]), the electron configuration is [Ar] 3d5 4s1, not [Ar] 3d4 4s2 as the Aufbau Principle would predict.

Applications

The Aufbau Principle is used extensively in chemistry and physics to predict the electron configuration of atoms, which in turn helps to explain chemical reactivity and the formation of bonds between atoms.

See also

• Hund's rule
• Pauli Exclusion Principle
• Quantum mechanics
• Electron configuration

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Aufbau principle gallery

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  Aufbau animated

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  Aufbau Principle diagram

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  Atomic orbitals as triangles

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  Sommerfeld ellipses