Orbital teaching sculpture beside a chemistry notebook, shown as a conceptual illustration. Original AI-generated editorial artwork produced for GreenCalc.
Sodium has eleven electrons when neutral. Writing “eleven” does not tell us how they occupy subshells. Its configuration, 1s² 2s² 2p⁶ 3s¹, does: two electrons in 1s, two in 2s, six in 2p and one in 3s.
Read the notation one piece at a time
The leading number identifies the principal shell. The letter identifies the subshell, and the superscript gives its electron population. The capacities are two for s, six for p, ten for d and fourteen for f. A neutral atom's populations must add up to its atomic number.
The Electron Configuration Calculator reports full configurations, noble-gas notation and shell totals for neutral atoms from hydrogen through uranium. The display orders occupied subshells by principal shell; this is a notation choice, not a claim that the printed order is an experimental timeline of electron addition.
Filling rules are a starting model
The Aufbau approach places electrons into lower-energy available orbitals first. The Pauli exclusion principle limits an orbital to two electrons with opposite spin, while Hund's rule describes occupancy of degenerate orbitals. A subshell population such as 2p³ does not itself draw the individual orbital boxes.
A common diagonal filling sequence is useful for introductory work. It is not sufficient for every ground-state atom. Electron interactions and closely spaced subshell energies produce exceptions, so the calculator applies explicit occupancy corrections.
Chromium, copper and palladium
Chromium is conventionally written [Ar] 3d⁵ 4s¹, rather than the simple filling prediction [Ar] 3d⁴ 4s². Copper is [Ar] 3d¹⁰ 4s¹. Palladium is [Kr] 4d¹⁰, with no occupied 5s subshell in the neutral ground-state configuration used here.
Explanations based on half-filled and filled subshells can help recall some examples. They are not a universal algorithm for all exceptions. Use evaluated configurations for a specific element instead of moving electrons whenever a pattern looks attractive.
Compressing a configuration
Noble-gas notation replaces a filled inner configuration with the symbol of the preceding noble gas in square brackets. Sodium becomes [Ne] 3s¹. The bracket does not add a neon atom: it represents ten inner electrons.
Shell totals answer another question. Sodium's totals are 2, 8, 1. They are useful for checking counts, but they omit the subshell information required by screening calculations.
Neutral atoms and ions need separate treatment
Do not obtain every ion configuration by deleting the rightmost printed term. Transition-metal ionisation is a familiar reason that such a shortcut fails. This tool deliberately reports neutral atoms only. Use the Atom Calculator for an ion's total electron count, without treating that count as a full configuration.
To use subshell occupancy in a screening model, open the Effective Nuclear Charge Calculator and read the Slater's rules worked guide. Explore bonding next through the Chemistry calculators.
By GreenCalc · Updated 2026-10-21
Frequently asked questions
Are ions supported?
No. This version reports neutral ground-state configurations for H through U.
Are chromium and copper exceptions included?
Yes. Explicit corrections include chromium, copper, palladium and other recognised H–U ground-state exceptions.