Short Answer
Hund’s rule of maximum multiplicity states that electrons occupy degenerate orbitals singly first, with parallel spins, before pairing up. Degenerate orbitals are orbitals in the same subshell that have the same energy, such as the three p-orbitals in a p-subshell.
This rule ensures that electrons are arranged to maximize total spin, which minimizes electron repulsion and stabilizes the atom. Hund’s rule helps in predicting electron configuration, orbital filling, and magnetic properties of atoms and ions.
Detailed Explanation :
Hund’s Rule of Maximum Multiplicity
Hund’s rule, formulated by Friedrich Hund, is a principle in quantum chemistry used to determine the most stable arrangement of electrons in degenerate orbitals. Degenerate orbitals are orbitals within the same subshell (such as p, d, or f) that have the same energy.
Statement of the Rule
The rule can be stated as:
- Every orbital in a subshell is singly occupied with one electron before any orbital is doubly occupied.
- All singly occupied electrons have the same spin direction (parallel spins).
This arrangement produces the maximum multiplicity, meaning the maximum number of unpaired electrons with the same spin.
Reason Behind Hund’s Rule
The principle is based on electron-electron repulsion and stability:
- Electrons are negatively charged and repel each other.
- Placing electrons singly in separate orbitals reduces repulsion between them.
- Parallel spins increase the total spin of the atom, which lowers energy and stabilizes the configuration.
By following Hund’s rule, atoms achieve the most energetically favorable electron arrangement.
Examples
- Carbon (C, atomic number 6):
Electron configuration: 1s² 2s² 2p²- 2p orbitals: px, py, pz
- According to Hund’s rule, the two electrons occupy px and py singly, both with parallel spins, rather than pairing in the same orbital.
- Oxygen (O, atomic number 8):
Electron configuration: 1s² 2s² 2p⁴- 2p orbitals: px², py¹, pz¹
- Electrons first occupy each orbital singly, then the next electrons pair in px.
- Nitrogen (N, atomic number 7):
Electron configuration: 1s² 2s² 2p³- 2p orbitals: px¹, py¹, pz¹
- All three electrons occupy separate orbitals with parallel spins for maximum stability.
Significance of Hund’s Rule
- Determines Electron Configuration:
Hund’s rule helps in filling degenerate orbitals correctly, ensuring accurate electron configurations. - Explains Magnetic Properties:
- Atoms with unpaired electrons are paramagnetic.
- Atoms with all paired electrons are diamagnetic.
Hund’s rule helps predict whether an atom is magnetic.
- Reduces Electron Repulsion:
By occupying orbitals singly first, electron-electron repulsion is minimized, making the atom more stable. - Foundation for Chemistry:
Hund’s rule is essential in understanding bonding, molecular structure, and reactivity in chemistry.
Relation to Other Principles
- Pauli Exclusion Principle: No two electrons in the same atom can have the same set of four quantum numbers.
- Aufbau Principle: Electrons fill orbitals starting from the lowest energy.
- Hund’s rule works with these principles to define the complete electron configuration.
Visual Representation
- p-orbitals (degenerate) can hold 6 electrons:
- 1st three electrons occupy px, py, pz singly, all parallel spin.
- Next three electrons pair up in the same orbitals.
This pattern ensures maximum multiplicity and minimum energy.
Conclusion
Hund’s rule of maximum multiplicity states that electrons occupy degenerate orbitals singly with parallel spins before pairing up. This arrangement reduces electron repulsion, maximizes total spin, and provides stability to atoms. It is essential for predicting electron configuration, magnetic properties, and chemical behavior, making it a fundamental principle in modern atomic theory and quantum chemistry.