Quarks are represented as octonions with eight components encoding physical properties. Due to the non-associativity of octonion multiplication, closure angles between triple products determine geometric stability: the proton undershoots the tetrahedral angle by 1.1° (stable), while the neutron overshoots by 6.5° (unstable). The neutron–proton mass difference is derived as 1.305 MeV (observed 1.293 MeV, 0.9% error) and the geometric escape toll as 1.218 MeV (observed 1.217 MeV, 0.15% error). No W boson, no Feynman diagrams, no free parameters.