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Orbital Configuration Simulator

Orbital box diagram

↑ spin-up electron↓ paired electronEach box is one orbital

How to use this simulation

Choose an element, or drag the atomic number, and the diagram fills orbitals from the bottom of the page upward. Each box is one orbital and holds at most two electrons. The s subshell has one box, p has three, d has five and f has seven. The text under the diagram gives the full configuration and the noble-gas shorthand.

  1. Start at hydrogen and step through the first ten elements. Check that 1s fills before 2s, and that 2p has three boxes.
  2. Stop on carbon, nitrogen and oxygen. Nitrogen should show three unpaired electrons, one in each 2p box, before oxygen starts pairing. That is Hund’s rule.
  3. Jump to chromium (24) and copper (29). A naive filling would give chromium 4s² 3d⁴ and copper 4s² 3d⁹. The diagram shows the observed 4s¹ 3d⁵ and 4s¹ 3d¹⁰ instead.
  4. Compare the full configuration with the noble-gas line. For calcium the shorthand is [Ar] followed by the electrons added after argon. The period and block readouts follow the highest occupied shell and the last subshell.

Key ideas

Aufbau order

Electrons occupy the lowest available energy level first. The order drawn here is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p. The 4s subshell is filled before 3d, which is why 4s sits below 3d on the diagram. The label “energy increases” points up the page.

Hund’s rule and the Pauli principle

Within a subshell, electrons occupy different orbitals with parallel spins before any orbital receives a second electron. An orbital then holds at most two electrons, and those two have opposite spins. The arrows in a box are that pair.

The familiar exceptions

Chromium and copper are the exceptions GCSE and A level expect: a half-full or full 3d subshell, with only one 4s electron, is lower in energy than the naive filling. The simulator also draws several later observed exceptions, up to ytterbium, including molybdenum, silver and palladium. If a configuration surprises you, compare it with the naive order before assuming the diagram is wrong.

Noble-gas shorthand, block and magnetism

The shorthand keeps the previous noble-gas core in brackets and writes only the outer electrons. The block is the type of the last subshell being filled. Unpaired electrons make the atom paramagnetic in this model; a completely paired configuration is labelled diamagnetic. That magnetic label is an A level and IB extension of the same orbital diagram.

Where this sits in the course

GCSE and IGCSE usually stop at electron shells. Subshells, orbital boxes, spin and the chromium and copper exceptions are A level and IB. The page is aimed at that part of the course, which is why the shell selector describes it as an advanced simulation.

What the model leaves out

The diagram is a ground-state teaching model for neutral atoms from hydrogen to ytterbium. It does not show excited states, ions, or the energy gap between subshells to scale. A full quantum treatment replaces the boxes with orbitals that have shapes; the boxes are the occupancy, not the shape.

More on the Chemistry simulations page, or the full SciSim library.