Hund's Rule: Why Electrons Are Anti-Social
Imagine a classroom with four empty chairs arranged in a row. Students enter one by one. Do they all rush to sit next to each other, or does each student pick a separate chair first? Obviously, they spread out — each takes an empty chair before anyone has to share. Electrons in an atom behave the same way, and that's the heart of Hund's rule.
The Intuition: Minimising Repulsion
Electrons are negatively charged. They repel each other. If you force two electrons into the same orbital (the same "chair"), they have to occupy the same region of space — that's a lot of repulsion. It costs energy. So nature's first instinct is to keep them apart.
But there's a second, subtler reason. Electrons also behave like tiny magnets (they have "spin"). Two electrons with the same spin direction (both spinning clockwise, say) actually avoid each other due to a quantum mechanical effect — they can't be in the same place with the same spin. This further reduces repulsion. So parallel spins (same direction) are energetically favourable.
The Precise Statement
Hund's Rule: When filling degenerate orbitals (orbitals of the same energy, like the three 2p orbitals), electrons occupy each orbital singly with parallel spins before any orbital gets a second electron.
"Degenerate" just means same energy. The three 2p orbitals (2px, 2py, 2pz) are degenerate. The five 3d orbitals are degenerate. Hund's rule governs how electrons fill these sets.
A Concrete Example: Nitrogen
Nitrogen has atomic number 7. Its electron configuration is 1s22s22p3. The 2p subshell has three orbitals and three electrons to place. Here's what happens:
- First electron goes into 2px (spin up, say).
- Second electron goes into 2py (spin up — parallel).
- Third electron goes into 2pz (spin up — all three parallel).
No pairing occurs. All three electrons have the same spin. This is the maximum multiplicity — the maximum number of unpaired electrons possible.
The total spin of the atom is maximised. For three unpaired electrons, total spin S=3×21=23. If you paired two electrons, you'd only have one unpaired electron (S=21), which is a higher-energy (less stable) arrangement.
What About Oxygen?
Oxygen has eight electrons: 1s22s22p4. Now we have four electrons for three 2p orbitals. Following Hund's rule:
- First three: one in each orbital, all spin up.
- Fourth electron: must pair up in one of the orbitals (say 2px), with spin down.
So oxygen has two unpaired electrons (in 2py and 2pz), not four. The pairing happens only after every orbital has one electron. …