Hund’s Rule and the Pauli Exclusion Principle are two principles in quantum mechanics that govern the behavior of electrons in atoms. Let’s talk more about the two.
What is Pauli Exclusion Principle?
The Pauli Exclusion Principle formulated by the Austrian physicist Wolfgang Pauli in 1925. It states that no two electrons (or other fermions) in an atom can have the same set of quantum numbers.
In other words, this principle dictates that within an atom, no two electrons can occupy the same quantum state simultaneously. Quantum numbers describe various properties of electrons, including their energy, orbital angular momentum, magnetic moment, and spin. Since electrons are fermions, they obey the Pauli Exclusion Principle, which prevents them from occupying identical quantum states.
The net effect of the Pauli Exclusion Principle is that it determines the electronic structure of atoms. It leads to the organization of electrons into different orbitals within an atom, with each orbital accommodating a maximum of two electrons with opposite spins. This principle is applied in understanding the stability and behavior of matter at the atomic level.
What you need to know about Pauli Exclusion Principle
- This principle states that no two electrons in an atom can have the same set of quantum numbers.
- Quantum numbers include the principal quantum number (n), the azimuthal quantum number (l), the magnetic quantum number (m), and the spin quantum number (s).
- The Pauli Exclusion Principle essentially means that electrons in an atom must have different spins if they occupy the same orbital. One electron must have spin “up” (designated as +1/2) and the other must have spin “down” (designated as -1/2).
- This principle is a fundamental rule governing the behavior of fermions, which include electrons.
- A large number of physical behaviors, including the structure of atoms’ electron shells and how they share electrons, are explained by the Pauli exclusion principle.
What is Hund’s Rule?
Hund’s Rule, named after the German physicist Friedrich Hund, is a principle that describes the arrangement of electrons within subshells (or orbitals) of an atom. It specifically deals with the distribution of electrons in degenerate orbitals, which are orbitals of the same energy level.
Hund’s Rule states that electrons will occupy empty orbitals within the same subshell before they start pairing up in orbitals. In other words, when filling degenerate orbitals (orbitals of the same energy), electrons will first occupy separate orbitals with parallel spins before they begin to pair up with opposite spins.
What you need to know about Hund’s Rule
- Hund’s Rule deals specifically with the arrangement of electrons within subshells (e.g., within the same energy level and orbital type).
- It states that electrons will occupy empty orbitals within the same subshell before they pair up. In other words, electrons will maximize their spin alignment (parallel spins) within a subshell before they begin to pair up (antiparallel spins).
- This rule helps to minimize electron-electron repulsion within a subshell, which leads to a more stable electron configuration.
- Hund’s Rule is often invoked when determining the electron configurations of atoms and ions.
- The lone pair of electrons in an atom and the valency can be determined using Hund’s rule.
Hund’s Rule vs Pauli Exclusion Principle
| Basis | Hund’s Rule | Pauli Exclusion Principle |
| Governs | Electron arrangement within subshells. | Electron uniqueness within an atom. |
| Focus | Maximizes total spin within a subshell | Ensures no two electrons have the same quantum numbers |
| Concerns | Electron spin orientation | Quantum numbers of electrons |
| Application | Determines electron configurations | Defines electron behavior in atoms |
| Principle | Electrons occupy empty orbitals before pairing up | No two electrons can have the same set of quantum numbers |
| Role in Stability | Maximizes stability by minimizing electron-electron repulsion | Ensures stable electronic configurations |
| Impact on Spectral Lines | Affects the fine structure splitting in spectral lines | Governs allowed transitions in atomic spectra |
| Quantum Mechanical Basis | Based on minimizing energy due to electron-electron repulsion | Arises from the symmetry requirements of wave functions |
Key Takeaways
- According to Hund’s Rule, if there are two or more degenerate orbitals (orbitals with the same energy) available, one electron must enter each one until they are all half full before pairing up. Because every partially filled electron in an orbital of a subshell has the same spin, the orbital posses maximum stability.
- According to the Pauli exclusion principle, no two electrons in a single atom will have the same set of quantum numbers (n, l, ml, and ms). Simply said, each electron should have or exist in its own unique state (singlet state). The two electrons in the same orbital must have opposing spins or be in an antiparallel configuration.