text: The Oddo-Harkins rule for elements from C  to  Fe is explained by the alpha process of stellar nucleosynthesis. The process involves the fusion of alpha particles (helium 4 nuclei) under high temperature and pressure within the stellar environment. Each step in the alpha process adds two protons (and two neutrons) favoring synthesis of even numbered elements. Carbon itself is a product of a triple alpha process from Helium, a process that skips Li, Be, and B. These nuclides (including helium-3) are produced by cosmic ray spallation a type of nuclear fission in which cosmic rays impact larger isotopes and fragment them. Spallation does not require high temperature and pressure of the stellar environment but can occur on Earth.  Though the lighter products of spallation are relatively rare, the odd mass number isotopes in this class occur in greater relative abundance compared to even number isotopes, in contravention of the Oddo-Harkins rule.