Oxygen (O, Z = 8)
Fill the subshells in Aufbau order until the running total reaches Z = 8:
- 1s² → 2
- 2s² → 4
- 2p⁴ → 8
Full configuration:
1s² 2s² 2p⁴
Noble-gas notation:
[He] 2s² 2p⁴
Unpaired electrons: 2
Open this example in the toolTool 03
Electron configuration calculator
Choose any element from hydrogen to oganesson to see its ground-state electron configuration in full and noble-gas notation, with an orbital box diagram. Where the real atom departs from the Aufbau order, as chromium and copper do, the page says so. A trainer below lets you practise.
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- Full configuration
- Noble-gas notation
Orbital diagram
Electron configuration trainer
Fill the subshells in the right order. The Aufbau diagonal is drawn beside you until you no longer need it.
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Worked examples
Iron (Fe, Z = 26)
Fill the subshells in Aufbau order until the running total reaches Z = 26:
- 1s² → 2
- 2s² → 4
- 2p⁶ → 10
- 3s² → 12
- 3p⁶ → 18
- 4s² → 20
- 3d⁶ → 26
Full configuration:
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶
Noble-gas notation:
[Ar] 3d⁶ 4s²
Unpaired electrons: 4
Open this example in the toolCopper (Cu, Z = 29)
Fill the subshells in Aufbau order until the running total reaches Z = 29:
- 1s² → 2
- 2s² → 4
- 2p⁶ → 10
- 3s² → 12
- 3p⁶ → 18
- 4s² → 20
- 3d⁹ → 29
Copper breaks the pattern: the Aufbau order predicts [Ar] 3d⁹ 4s², but one 4s electron moves into 3d, because a completely filled 3d subshell gives a lower energy. The ground state is [Ar] 3d¹⁰ 4s¹.
Full configuration:
1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰
Noble-gas notation:
[Ar] 3d¹⁰ 4s¹
Unpaired electrons: 1
Open this example in the toolHow it works
Why does 4s fill before 3d?
Because energy, not shell number, decides the order. The Madelung rule says subshells fill by increasing n + l: 4s has n+l = 4, while 3d has n+l = 5, so 4s wins. Once electrons are actually in the atom the picture inverts — in a neutral transition metal 3d sits below 4s, which is why ions like Fe²⁺ lose their 4s electrons first, not their 3d ones. Chromium and copper break the pattern outright, taking a half-full or full d shell over a full s shell.
Where the configurations come from
The configuration shown for each element is the ground state in the site's reference data, not a computed guess, so the known exceptions appear as they really are. The Aufbau prediction is worked out separately with the n + l rule and shown beside it whenever the two differ. The orbital diagram fills each subshell by Hund's rule: every orbital takes one electron before any takes a second.
Limits
- It describes neutral atoms in their ground state, not ions or excited states.
- From rutherfordium (element 104) onward the configurations are theoretical predictions, because too few atoms have been made to measure them.
- The trainer asks about elements up to strontium (element 38), where the filling order is the one usually taught.
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