October 1, 2025

What Is The Aufbau Principle

Q: Are there any exceptions to the Aufbau principle?

Yes, there are exceptions, particularly for transition metals like chromium (Cr) and copper (Cu). A half-filled or completely filled d-subshell is more stable, so an electron from the 4s orbital will move to the 3d orbital to achieve this stability.

Q: What other rules govern electron configuration?

The Aufbau principle works with two other key rules: the Pauli Exclusion Principle, which states that no two electrons can have the same four quantum numbers, and Hund's Rule, which states that electrons will occupy orbitals of the same energy singly before pairing up.

Q: How can you remember the order of orbital filling?

A common method is to use a diagonal rule chart (or Madelung rule diagram). You write the orbitals in columns (1s, 2s, 3s, etc.) and then draw diagonal arrows through them to show the correct filling order (1s -> 2s -> 2p -> 3s -> 3p -> 4s -> 3d...).

Q: Does the Aufbau principle apply to ions?

Yes. To find the configuration of an anion (negative ion), you add electrons to the next available orbital following the Aufbau order. For a cation (positive ion), you remove electrons from the orbital with the highest principal quantum number (n) first.

A clear explanation of the Aufbau principle, which describes how electrons fill atomic orbitals in order of increasing energy. Includes rules and examples.

Have More Questions →

Understanding the Aufbau Principle

The Aufbau principle, from the German word for 'building up', states that in the ground state of an atom or ion, electrons fill the lowest available energy atomic orbitals before occupying higher energy levels. This creates the most stable electron configuration.

Section 2: The Order of Orbital Filling

Electrons are added one by one to orbitals in a specific sequence of increasing energy. This order is generally 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on. A key point is that the 4s orbital has a lower energy level than the 3d orbital, so it is filled first, which explains the structure of the periodic table's transition metals.

Section 3: A Practical Example (Carbon)

Consider a carbon atom, which has 6 electrons. According to the Aufbau principle, the first two electrons fill the 1s orbital (1s²). The next two fill the 2s orbital (2s²). The remaining two electrons go into the 2p orbital (2p²). Therefore, the ground-state electron configuration for carbon is 1s²2s²2p².

Section 4: Importance in Chemistry

The Aufbau principle is fundamental for predicting the electron configuration of any element. This configuration, in turn, determines an element's chemical properties, including its reactivity, bonding behavior, and placement on the periodic table. It is used in conjunction with the Pauli exclusion principle and Hund's rule to fully describe electron arrangements.

Frequently Asked Questions

Are there any exceptions to the Aufbau principle?

What other rules govern electron configuration?

How can you remember the order of orbital filling?

Does the Aufbau principle apply to ions?