# Pauli exclusion principle and Hund's rule

Pauli: no two electrons in an atom can have all four quantum numbers the same, so an orbital holds at most two, with opposite spins. Hund: in equal-energy orbitals, electrons spread out singly before pairing up.

Chemistry (university) · Part 01: Atomic structure and periodic trends · Card 7 of 10 · Last updated 2026-10-19

## Analogy

Like people on a bus: each takes an empty double seat alone first, and only shares when every seat is taken.

## Example

**Nitrogen 2p³**

↑ _ _ is wrong
↑ ↑ ↑ is right
Three 2p orbitals, one electron each,
all spinning the same way

## In practice

Hund's rule explains why oxygen has two unpaired electrons and is attracted to a magnet when cooled to a liquid.

## Practice

### Flashcards

- **State the Pauli exclusion principle.** No two electrons in an atom share all four quantum numbers, so an orbital holds at most two, with opposite spins.

### Interview questions

**Write the electron configuration of Fe and Fe³⁺ and explain which electrons are lost first.**

- Fe is [Ar] 4s² 3d⁶
- 4s electrons are lost before 3d
- Fe³⁺ is [Ar] 3d⁵
- Half-filled d subshell is stable

**Why does the first ionization energy of oxygen fall slightly below that of nitrogen?**

- Nitrogen has three unpaired 2p electrons (half-full)
- Oxygen has one paired 2p orbital
- Repulsion between paired electrons makes one easier to remove

## Questions people ask

### Why do electrons avoid pairing in the same orbital?

Two electrons in one orbital sit close together and repel each other strongly. Placing them in separate equal-energy orbitals keeps them further apart, so the atom has lower energy. They only pair up when no empty orbital of that energy is left.

### What does unpaired electron mean for magnetism?

Each electron acts like a tiny magnet. Paired electrons have opposite spins, so their magnetic effects cancel. Atoms with unpaired electrons are paramagnetic, which means they are pulled into a magnetic field. Atoms with all electrons paired are diamagnetic and are weakly pushed away.

## Related concepts

- [Electron configuration](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-1/chemistry-university-part-1-concept-6)
- [Ionization energy](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-1/chemistry-university-part-1-concept-10)

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