Aufbau principle

Aufbau Principle

The Aufbau Principle is a fundamental guideline in atomic structure that explains how electrons are filled into atomic orbitals. The term “Aufbau” comes from a German word meaning “building up,” and this principle essentially describes how atoms are built up by adding one electron at a time.

What Does the Aufbau Principle State?

Electrons are added to atomic orbitals beginning with those of the lowest energy, gradually filling higher energy levels. This approach is useful for determining the ground-state electronic configuration of an element.

Order of Orbital Filling

The order in which orbitals are filled is not strictly sequential by shell number (n = 1, 2, 3, etc.) but follows the increasing order of (n + l) values:

• If two orbitals have the same (n + l) value, the one with the lower n value is filled first.
• This is known as the (n + l) rule or Bohr-Bury rule.

Orbital filling sequence:

1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s ...

Example

Electronic Configuration of Oxygen (Atomic Number = 8):
1s² 2s² 2p⁴
Electrons are added one by one in the increasing energy order of orbitals.

Exceptions to the Aufbau Principle

Some elements like Chromium (Cr) and Copper (Cu) show deviations from the expected configuration due to additional stability from half-filled and fully filled d-subshells:

• Cr: [Ar] 3d⁵ 4s¹ (instead of 3d⁴ 4s²)
• Cu: [Ar] 3d¹⁰ 4s¹ (instead of 3d⁹ 4s²)

Importance in Chemistry

• Helps determine the correct electronic configuration of atoms and ions.
• Essential for understanding periodic trends like atomic size, ionization energy, and chemical reactivity.
• Helps explain how chemical bonds form and how elements are arranged in the periodic table.

The Aufbau Principle plays a fundamental role in explaining the arrangement of electrons within an atom. By filling orbitals in a defined energy order, it allows us to predict the electronic structure of elements, which in turn determines their chemical properties.