Chemistry
Electron Configuration
Electron configuration describes how electrons fill the orbitals of an atom. Three rules govern the filling: Aufbau (fill lowest energy first), Pauli exclusion (max 2 electrons per orbital with opposite spins), Hund's rule (fill degenerate orbitals singly before pairing).
How to Approach Electron Configuration
List the orbitals in fill order
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d. Note 4s fills BEFORE 3d (Aufbau order ≠ shell order).
Distribute electrons by atomic number
Each orbital holds 2 electrons (Pauli). s holds 2, p holds 6, d holds 10, f holds 14. Fill in the Aufbau order until you've placed all Z electrons.
Apply Hund's rule and exceptions
Within a subshell (e.g. 2p), fill each orbital with one electron before pairing. Two exceptions: Cr (3d⁵ 4s¹ — half-filled stability) and Cu (3d¹⁰ 4s¹ — full-filled stability).
Frequently Asked Questions
What's the difference between configuration and noble gas notation?+
Full configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ ... for every electron. Noble gas: [Ar] 4s² 3d¹⁰ ... — abbreviated using the previous noble gas.
Why is 4s before 3d?+
Because the 4s orbital is slightly lower in energy than 3d for atoms beyond potassium. After both are partially filled, 3d drops below 4s — that's why ions of transition metals lose 4s electrons first.
What are the exceptions and why?+
Cr is [Ar] 3d⁵ 4s¹ (not 3d⁴ 4s²) and Cu is [Ar] 3d¹⁰ 4s¹ (not 3d⁹ 4s²). Half-filled and fully-filled d subshells are unusually stable, so the atom prefers them to a 4s² configuration.
Related Topics
More step-by-step guides in Chemistry and adjacent subjects.
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