The Exclusion Principle and Multi-Electron Atoms
No two electrons in an atom share all four quantum numbers, so electrons stack into shells instead of collapsing into the lowest state. The layout of the periodic table follows from that stacking rule.
What a learner can do afterwards
- Uses the four quantum numbers and the exclusion principle to fill the levels of a light atom
- Explains why chemical behaviour repeats along a period
- Describes how screening changes the order in which sub-shells fill
1 · Read
Every electron in an atom carries four labels: n for its shell, l for its subshell shape, m for its orientation, and ms for its spin. The exclusion rule says no two electrons in one atom may share all four labels at once. This single rule stops every electron from sinking into the lowest slot.
Build carbon, which has six electrons. Two fill the first shell as 1s2, two fill 2s2, and the last two go into 2p, giving 1s2 2s2 2p2. Counting the labels shows the first shell holds 2, the second holds 8, and the third holds 18.
Atoms with the same outer filling behave alike, so each column of the periodic table is a family. When the outer s and p subshells fill, the next row starts the pattern again, which is why chemistry repeats from one row to the next. The layout of the table follows from the stacking rule.
Inner electrons screen the nucleus and soften its pull on outer ones, and this shuffles the fill order. The 4s subshell fills before 3d, which is why the transition metals appear. Always fill lowest energy first, not strict shell order.
Four labels per electron, no two alike, so shells fill in an order that repeats across the table.
Readings written for this dataset.
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