Electrons in boxes and free radicals

• Each box is one orbital: s is one box, p is three, d is five.
• Each arrow is one electron. The arrow direction shows its spin.
• Two electrons in one box must have opposite spins: [↑↓], a spin pair.
• Label each group of boxes with its sub-shell (2s, 2p, 3d …). You can start with a noble-gas core such as [Ar].

1. Fill sub-shells in energy order (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p).
2. Within a sub-shell, put one electron in each box first, all with the same spin (parallel).
3. Only then start pairing up.

Why rule 2? Two electrons in the same orbital repel strongly (spin-pair repulsion). Keeping them in separate orbitals of the same energy keeps them apart and gives a lower total energy.