How this calculator works
Fill subshells in their usual energy sequence and apply neutral ground-state exceptions through lawrencium. The displayed sequence is then sorted by principal shell. A prior noble gas replaces the closed core in shorthand. Configurations above Z = 103 are labelled Aufbau predictions because relativistic effects can change their ground states. Atomic weights come from 2021, separately from the older NIST configuration reference.
Formula
Subshell capacities: s = 2, p = 6, d = 10, f = 14. The sum of all occupancies equals Z for a neutral atom.
Worked example
Neon is 1s2 2s2 2p6, or [He] 2s2 2p6. Chromium is [Ar] 3d5 4s1, an exception to naive filling.
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Frequently asked questions
Does this handle ions?
This page describes neutral atoms. Ionization can remove outer-shell electrons in a different order from filling.
Why are 3d and 4s reordered?
Filling order describes approximate energies. Display order groups subshells by principal shell.
Why is there no atomic weight for some elements?
Many radioactive elements have no IUPAC standard atomic weight. Their configurations still have a defined or predicted electron count.