text: The elemental basis of the Oddo-Harkins has direct roots in the isotopic compositions of the elements. While even atomic-numbered elements are more abundant than odd, the spirit of Oddo-Harkins extends to the most abundant isotopes as well. Isotopes containing an equal number of protons and neutrons are the most abundant. These include , , , ,  ,  , , and  . Seven of the eight are alpha nuclides containing whole number multiples of He-4 nuclei ( is the exception). Two of the eight (  and ) contain  magic numbers of either protons or neutrons (2, 8, 20, 28, 50, 82, and 126) and are therefore predicted by the nuclear shell model to be unusually abundant. The high abundances of the remaining six (, ,  ,  , , and  ) are not predicted by the shell model.  "That nuclei of this type are unusually abundant indicates that the excess stability must have played a part in the process of the creation of elements," stated Maria Goeppert Mayer in her acceptance lecture for the Nobel Prize in Physics in 1963 for discoveries concerning nuclear shell structure.
  The Oddo–Harkins rule may suggest that elements with odd atomic numbers have a single, unpaired proton and may swiftly capture another in order to achieve an even atomic number and proton parity. Protons are paired in elements with even atomic numbers, with each member of the pair balancing the spin of the other, thus enhancing nucleon stability. A challenge to this explanation is posed by , which is highly abundant in spite of having an unpaired proton. Additionally, even parity isotopes that have exactly two more neutrons than protons are not particularly abundant despite their even parity. Each of the light elements oxygen, neon, magnesium, silicon, and sulfur, have two isotopes with even isospin (nucleon) parity.  As shown in the plot above, the isotope with an equal number of protons and neutrons is one to two orders of magnitude more abundant than the isotope with even parity but two additional neutrons. This may leave open the role of parity in abundance. The structural or subatomic basis of the unusual abundances of equinucleonic isotopes in baryonic matter is one of the simplest and most profound unsolved mysteries of the atomic nucleus.