Chemistry / Unit 5 Chem 0862 worked example, 60 to 90 seconds
Writing an Electron Configuration
The idea
Writing an atom's ground-state electron configuration means filling its orbitals in Aufbau order, respecting the Pauli exclusion principle's two-electron limit per orbital and Hund's rule for any degenerate subshell, until every electron matching that element's atomic number is placed.
In practice
Building phosphorus, atomic number 15, fills 1s² then 2s² then 2p⁶, using up ten electrons, then 3s² and finally 3p³, placing the three remaining electrons singly across the three 3p orbitals in accordance with Hund's rule. The full configuration reads 1s²2s²2p⁶3s²3p³, or [Ne]3s²3p³ in core-abbreviated form, and the superscripts sum to 15, matching phosphorus's atomic number.
Where people slip
Pairing two 3p electrons together before placing one electron in each of the three 3p orbitals violates Hund's rule, even though the total electron count still comes out correct.
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