Hund’s rule of maximum multiplicity is a rule used to predict how electrons are arranged in orbitals of the same energy (degenerate orbitals, like the three p orbitals, five d orbitals, or seven f orbitals).
It states:
Electrons fill degenerate orbitals singly first, with parallel spins, before pairing up.
In simple words:
- If several orbitals of the same energy are available, each orbital gets one electron before any orbital gets a second electron.
- These single electrons in different orbitals have the same spin direction (all spin-up or all spin-down).
Example:
For the p orbitals (px, py, pz):
- If there are 3 electrons, they will occupy px, py, and pz separately, all with parallel spins ↑ ↑ ↑.
- Only when more than 3 electrons are present do they start pairing.
Importance:
- This minimizes electron–electron repulsion.
- Makes the atom more stable.
- Helps explain magnetic properties of elements (like why oxygen is paramagnetic).