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Why don’t all of an atom’s electrons crowd into the lowest orbit? In 1925 Wolfgang Pauli proposed the exclusion principle: no two identical fermions can share the same quantum state. Swapping two electrons flips the sign of their joint wavefunction, so a doubly occupied state cancels to zero. That rule builds the periodic table. A 3" pixel-art physicist sticker; equation, symbols and sources in the physics panel below.

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Size3″ × 3″

3″ × 3″ kiss-cut sticker.

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Returns & replacements21 days

Every item is printed to order, so we can’t accept returns or exchanges for change of mind, or if you ordered the wrong size or colour.

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The physics

Wolfgang Pauli wrote down the exclusion principle in 1925: no two identical fermions may share the same set of quantum numbers.

In wave-language that is antisymmetry. Swap two electrons and the joint wavefunction changes sign. If both try to occupy the exact same state, the wavefunction equals its own negative, so it must be zero — the configuration is forbidden.

On the sticker the blackboard shows one filled 1s slot (spin up) and a blocked attempt to put a second electron in the same slot. Opposite spins in the same orbital are still allowed; a third copy of either is not.

Exclusion shapes the periodic table and the stability of ordinary matter.

One per state.

Equations

\[\psi(1,2)=\frac1{\sqrt2}\bigl[\phi_a(1)\phi_b(2)-\phi_a(2)\phi_b(1)\bigr]\]
\[\psi\equiv 0\quad\text{when }a=b\]
Symbols
SymbolMeaningUnit
\(ψ\) two-particle antisymmetrised wavefunction —
\(φ_a, φ_b\) single-particle orbitals (including spin) —
\(a, b\) full sets of quantum numbers (n,ℓ,m_ℓ,m_s) —

Sources

  1. Pauli (1925) Über den Zusammenhang des Abschlusses der Elektronengruppen im Atom mit der Komplexstruktur der Spektren, Z. Phys. 31, 765
  2. Dirac (1926) On the theory of quantum mechanics, Proc. R. Soc. Lond. A 112, 661
  3. Halliday, Resnick & Krane, Physics, Ch. on the Pauli exclusion principle
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