Define Hund's rule.

Define Hund's rule.

Hund's rule governs the distribution of electrons in orbitals of the same energy level. It says, "Electrons pairing in any orbital of the same sub-level will not occur until each orbital contains one electron".

Hund's Rule Explained

Hund's rule, sometimes referred to as Hund's rule of maximum multiplicity, is a fundamental principle in atomic and molecular chemistry that helps predict the electron configuration and ground state of atoms and molecules. It deals specifically with how electrons arrange themselves within orbitals of equal energy (degenerate orbitals).

Here's the core idea

For a given electron configuration, the state with the highest total spin multiplicity will have the lowest energy.

This means two things:

  1. Electrons will occupy all available orbitals within a subshell singly before pairing up in any one orbital. Imagine each orbital as a little apartment. Electrons prefer having their own "apartment" before sharing with a roommate.
  2. When occupying singly, all electrons in those orbitals will have the same spin direction. They can either all be "up" or all be "down", but not a mix. This minimizes electron-electron repulsion, leading to a lower energy state.

Hund's rule is essential for

  • Constructing accurate electron configurations for atoms and molecules.
  • Understanding and interpreting atomic and molecular spectra.
  • Predicting the magnetic properties of atoms and molecules.

It's important to note that Hund's rule is one of three rules used to determine the most stable electron configuration. The other two, called the Aufbau principle and the Pauli exclusion principle, guide filling orbitals in increasing energy order and ensure no two electrons share the same set of quantum numbers, respectively.

Remember, Hund's rule applies only when orbitals have the same energy and applies mainly to open subshells (not completely filled or empty ones).

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