The Aufbau principle

Atomic Theory Contributions:

• Determines the electron configuration of atoms, molecules or ions

• Atoms are built up by continually adding electrons.

• The term "Aufbau" is German for "building up," reflecting how electrons occupy orbitals starting from the lowest energy level to higher energy levels.

  Key Points of the Aufbau Principle:

  1. Electron Configuration:

     • Electrons fill orbitals starting with the lowest energy level and proceed to higher energy levels.

     • This "building up" process follows a specific order based on the increasing energy of orbitals.

  2. Order of Filling:

     • The order in which orbitals are filled is determined by their relative energy levels.

     • The sequence can be remembered using the following list or diagram, which shows the order of orbitals from lowest to highest energy:

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       1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p

  3. Rules for Electron Filling:

     • Pauli Exclusion Principle: Each orbital can hold a maximum of two electrons, which must have opposite spins.

     • Hund's Rule: When electrons fill degenerate orbitals (orbitals of the same energy), they will singly occupy each orbital with parallel spins before pairing up.

     • Aufbau Principle: Electrons occupy the lowest energy orbital available.

  Visualization of the Order of Filling:

  A useful way to remember the order of filling is using a diagonal rule diagram. Start from the top of each column and move diagonally downwards to determine the sequence:

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  1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p

  Example of Electron Configurations:

  1. Hydrogen (H):

     • Atomic number = 1

     • Electron configuration: 1s¹

  2. Carbon (C):

     • Atomic number = 6

     • Electron configuration: 1s² 2s² 2p²

  3. Iron (Fe):

     • Atomic number = 26

     • Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

  4. Neon (Ne):

     • Atomic number = 10

     • Electron configuration: 1s² 2s² 2p⁶

  Exceptions to the Aufbau Principle:

  While the Aufbau principle provides a general guideline, there are some exceptions, particularly among transition metals and heavier elements. These exceptions arise because the actual energy difference between some orbitals is very small, leading to configurations that maximize stability.

  1. Chromium (Cr):

     • Expected: [Ar] 4s² 3d⁴

     • Actual: [Ar] 4s¹ 3d⁵

  2. Copper (Cu):

     • Expected: [Ar] 4s² 3d⁹

     • Actual: [Ar] 4s¹ 3d¹⁰

  These exceptions occur because half-filled and fully filled d subshells provide extra stability.