Electron configuration: watch an atom fill

One electron at a time, across all 118 elements. Watch subshells fill, then see how their electrons add up to shells.

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Watch the atom fill

Calculated electron probability clouds, colored by shell. The shell totals below give the electron counts.

Watch the atom build electron by electron, in sync with the orbital boxes.

Cloud sizes and radial nodes come from orbital calculations. Dots sample probability; colors identify shells. The scale bar shows physical distances.

Probability of finding an electron at each distance from the nucleus.

A radial node is a distance where this orbital has zero probability. Other orbitals can fill the gap in the combined cloud. Orbitals belong to the final neutral atom and stay fixed during this filling lesson.

Subshells & orbital boxes

Read top to bottom in filling order. One box = one orbital; each arrow = one electron’s spin.

Shell totals

The number in 2p means shell n = 2; p identifies the subshell within it.

Bars compare occupancy with the shell capacity 2n². Calcium has 2, 8, 8, 2: 4s fills before 3d, so shell 3 is still below its capacity of 18.

Three rules to watch

Aufbau: use the usual subshell order as a starting point: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p. Some elements have different ground-state occupancies.
Hund: within a subshell, occupy the equal-energy orbitals singly with parallel spins before pairing.
Pauli: an orbital holds at most two electrons, with opposite spins.

This builds the selected neutral atom’s final configuration one contribution at a time; intermediate steps do not represent the ground states of ions or other elements. Arrows show spin states, and box spacing does not represent measured energies.

Reference: OpenStax · Electronic structure of atoms

Try these electron configurations

Nitrogen: 1s² 2s² 2p³ shows three singly occupied p orbitals. Oxygen: 1s² 2s² 2p⁴ adds the first pair. Calcium: [Ar] 4s² shows why the outer shell starts before 3d fills.