# Difference Between Aufbau Principle and Hund’s Rule

### What is Aufbau Principle?

The Aufbau Principle, also known as the building-up principle, is a concept that describes the order in which electrons fill atomic orbitals as an atom is built up. It states that electrons occupy the lowest energy orbitals available to them before filling higher energy levels.

According to the Aufbau Principle:

• Electrons fill orbitals starting from the lowest energy level and proceed to higher energy levels successively.
• Each electron occupies the lowest energy orbital available, given the Pauli exclusion principle, which states that no two electrons in an atom can have the same set of quantum numbers (meaning they must have different spins).
• The sequence in which orbitals are filled follows the order of increasing energy levels, typically determined by the principal quantum number (n), azimuthal quantum number (l), and magnetic quantum number (m).

For example, in the ground state electron configuration of carbon (C), which has 6 electrons, the Aufbau Principle dictates that the electrons fill the orbitals in the following order:

1s<sup>2</sup> 2s<sup>2</sup> 2p<sup>2</sup>

Here, the 1s orbital is filled first (lowest energy), followed by the 2s orbital, and then the 2p orbital.

### What is Hund’s Rule?

Hund’s Rule is a principle that describes the way electrons fill orbitals in an atom when multiple orbitals of the same energy level (degenerate orbitals) are available. It was formulated by German physicist Friedrich Hund in 1927. Hund’s Rule helps to explain the arrangement of electrons within subshells and the resulting electron configurations.

Hund’s Rule states:

• Electrons tend to occupy empty orbitals within a subshell singly with parallel spins before any orbital is doubly occupied. This means that electrons preferentially occupy different orbitals within a subshell rather than pairing up immediately.
• When filling degenerate orbitals, electrons will first occupy separate orbitals (if possible) and align their spins parallel to each other. This maximizes the total spin magnetic quantum number (ms) and leads to a lower energy configuration, as dictated by the Pauli exclusion principle.

### Key Takeaways

Aufbau principle

• Aufbau principle states that electrons fill atomic orbitals of an atom in a specific order, beginning with the lowest energy orbital and progressing to higher energy orbitals.
• Electrons occupy orbitals corresponding to increasing energy levels, following the sequence: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.
• Each orbital can hold a maximum of two electrons with opposite spins, according to the Pauli exclusion principle. Electrons in the same orbital must have opposite spins.
• Aufbau principle is important in determining the electron configuration of an atom, which represents the distribution of electrons among the various atomic orbitals.
• The arrangement of electrons based on aufbau principle influences the chemical behavior of atoms, determining their reactivity and ability to form chemical bonds.

Hund’s Rule

• Hund’s rule states that electrons occupy orbitals singly with parallel spins before pairing up. This maximizes the total spin of the electrons, making the atom or ion more stable.
• When multiple orbitals of the same energy level (degenerate orbitals) are available, electrons tend to occupy them singly with parallel spins, rather than pairing up immediately.
• Hund’s rule is applied when determining the electron configuration of atoms or ions, especially for elements with partially filled subshells.
• There are exceptions to Hund’s rule, particularly for transition metals and heavier elements, where the energy difference between orbitals can lead to deviations from the expected electron configuration.
• Hund’s rule influences the magnetic properties of materials, as the arrangement of unpaired electrons in degenerate orbitals can lead to paramagnetic behavior.