No two electrons in an atom can have the same set of four quantum numbers, so an orbital holds at most two electrons.
Practise Atomic Structure & Periodicity All Atomic Structure & Periodicity topics
The Pauli exclusion principle states that no two electrons in the same atom can have the same set of four quantum numbers. Because the spin quantum number has only two values, an orbital can hold at most two electrons, and they must have opposite spins. This principle is responsible for the shell structure of atoms, the layout of the periodic table and the stability of ordinary matter. Without it, all electrons would collapse into the lowest orbital. Wolfgang Pauli formulated the principle in 1925.
Key ideas: No two electrons in an atom can have the same four quantum numbers, so an orbital holds at most two electrons with opposite spins. Counting rule: shell n holds 2 n^2 electrons, subshell holds 2 (2 l + 1). It stems from the fact that electrons are fermions with antisymmetric wave functions.
Common mistakes and connections: Students think the principle says electrons must be paired, but it only limits the number to two, and Hund's rule states pairing is delayed. It is the reason atoms have size and matter is stable, and it explains the periodic table's period lengths 2, 8, 8, 18, 18, 32.
An orbital holds at most two electrons and they must have opposite spins, since no two electrons share all four quantum numbers. The interactive lesson shows this as a live animation or chart.
Neon has ten electrons, filling 1s2, 2s2 and three paired 2p orbitals, with no two electrons sharing the same four quantum numbers.
No two electrons in an atom can have the same set of four quantum numbers, so an orbital holds at most two electrons.
Pauli Exclusion Principle is a beginner topic in Atomic Structure & Periodicity. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Atomic Structure & Periodicity first: Radial and Angular Nodes, Electron Shells and Subshells, Aufbau Principle.
The next topics in Atomic Structure & Periodicity are Hund's Rule and Madelung Rule.
Yes. The interactive lesson includes a self-check question on Pauli Exclusion Principle, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Pauli Exclusion Principle is topic 62 of 115 in Atomic Structure & Periodicity.