IV.R135 — Why He-4 is tightly bound

The alpha-particle (He-4, Z=N=2) is the first doubly magic nucleus: two protons and two neutrons each fill the lowest winding mode (0,0) with both spin orientations, completely filling all four nucleon slots; the large gap to the next shell makes it exceptionally bound (B/A ~ 7.1 MeV) and the preferred alpha-decay product.