Hund's Rule
Definition and meaning of Hund's Rule in chemistry.
Hund's rule explains how electrons fill up atomic orbitals that have the exact same energy. It states that every orbital in a matching set must get one electron first. Only after every orbital has one electron can a second electron join a pair.
In more detail
Electrons are all negatively charged, so they naturally want to push away from each other. Forcing two electrons into the exact same small orbital creates a lot of stressful repulsion. To avoid this crowding, electrons spread out into empty rooms whenever they possibly can.
They also spin in the exact same direction when they are sitting by themselves. Having these parallel spins gives the whole atom a special kind of relaxing energy. Chemists call this calming effect exchange energy, and it makes the atom much more stable.
Once every single equal orbital has one spinning electron, the empty spaces are completely gone. The next arriving electrons must finally pair up with the ones that are already there. When they pair up, the new electrons must spin in the completely opposite direction.
This opposite spinning rule is strictly required by a law called the Pauli exclusion principle. Students often try to apply this spreading rule to every single orbital in the atom. However, Hund's rule only works for orbitals that share the exact same energy level.
It does not apply when electrons move between different orbital types like s and p. An electron will always fill a lower energy s orbital before a higher p orbital.
Key facts
| Formal name | Hund's rule of maximum multiplicity |
|---|---|
| Applies to | Degenerate orbitals (orbitals with equal energy) |
| Main goal | Minimizes electron repulsion |
| Spin requirement | Single electrons must have parallel spins |
| Next step | Electrons pair with opposite spins when full |
| Field | General Chemistry |
A nitrogen atom has a total of seven electrons to organize. Two electrons fill the 1s orbital and two fill the 2s orbital. This leaves exactly three electrons to fill the three equal 2p orbitals. Instead of putting two electrons in the first 2p orbital, they spread out. One electron goes into the 2px orbital, one into 2py, and one into 2pz. All three of these lonely electrons spin in the exact same parallel direction.
Frequently asked questions
Why do electrons prefer to occupy separate orbitals first?
Electrons are negatively charged, so they naturally repel each other. Spreading out into separate orbitals keeps them far apart and lowers the stress inside the atom.
Do electrons spinning in the same direction help the atom?
Yes, having parallel spins gives the atom a special stabilizing boost called exchange energy. This makes the entire atom much more relaxed and stable overall.
Does Hund's rule apply to every orbital in an atom?
No, it only applies to a set of orbitals that have the exact same energy. For example, it works for the three 2p orbitals, but not between the 2s and 2p orbitals.