The three rules behind every configuration
- Aufbau principle — electrons fill the lowest-energy subshell available: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → …
- Pauli exclusion principle — each orbital holds at most two electrons, with opposite spins.
- Hund's rule — within a subshell, electrons occupy empty orbitals singly before pairing up.
Subshell capacities: s holds 2, p holds 6, d holds 10, f holds 14. The notation 3p⁴ means “4 electrons in the p subshell of shell 3”.
Reading the results
The noble-gas shorthand compresses the core electrons into brackets: iron's 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶ becomes [Ar] 3d⁶ 4s². The electrons outside the brackets are the chemically interesting ones. Valence electrons — those in the highest shell — determine bonding; see Valence Electrons Explained.
About the exceptions
About twenty elements have measured ground states that differ from the naive Aufbau prediction, because half-filled (d⁵) and filled (d¹⁰) subshells are unusually stable. The classics are chromium ([Ar] 3d⁵ 4s¹) and copper ([Ar] 3d¹⁰ 4s¹). This calculator always shows the measured configuration and tells you when — and how — it differs from the prediction.