Seats

seat each electron. the hall has rules.

seat electrons on the benches of an atom, row by row: the lowest row first, alone before pairing, two to a bench; then Cr, Cu and ions like Fe3+

Electrons fill an atom from the lowest-energy row up, and 4s sits below 3d, so it fills first. In a row they take empty benches before pairing up (Hund), and two on a bench face opposite ways (Pauli). Ions lose 4s electrons first. Chromium and copper move one 4s electron into 3d, because a half-full or full 3d is more stable. Count the electrons sitting alone and the spin-only moment is the square root of n(n+2).

Concepts: chemistry, atomic-structure, order.

An interactive you play with one thumb, on Wundr.