Free interactive chemistry · SIM-ATOM-002

Orbital filling (Aufbau/Pauli/Hund)

Electrons fill orbital boxes one by one, following the Aufbau, Pauli and Hund rules.

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How to use this simulation

Choose an atom with the proton (atomic number) control.
Look at the “Orbital boxes” panel: each box holds up to two arrows (↑↓).
Step through the filling one electron at a time to see the order.
Add or remove electrons to see the configuration of ions.

What to notice

Order: 1s → 2s → 2p → 3s → 3p → 4s → 3d …
Hund: each box in a subshell gets one ↑ before any pairing.
Cr and Cu are exceptions — the panel flags them.

Try this

How many unpaired electrons does iron (Z = 26) have?

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