Oddo–Harkins rule
Generally, the relative abundance of an even atomic numbered element is roughly two orders of magnitude greater than the relative abundances of the immediately adjacent odd atomic numbered elements to either side.
[3][4] The Oddo–Harkins rule is true for all elements beginning with carbon produced by stellar nucleosynthesis but not true for the lightest elements below carbon produced by big bang nucleosynthesis and cosmic ray spallation.
[citation needed] All atoms heavier than hydrogen are formed in stars or supernovae through nucleosynthesis, when gravity, temperature and pressure reach levels high enough to fuse protons and neutrons together.
Harkins observed that elements with even atomic numbers (Z) were about 70 times more abundant than those with odd Z.
[5]: 385 This early work connecting geochemistry with nuclear physics and cosmology was greatly expanded by the Norwegian group created by Victor Goldschmidt.
[6]: 42 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.
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.
This postulate, however, does not apply to the universe's most abundant and simplest element: hydrogen, with an atomic number of 1.
This may be because, in its ionized form, a hydrogen atom becomes a single proton, of which it is theorized to have been one of the first major conglomerates of quarks during the initial second of the Universe's inflation period, following the Big Bang.
In this case, helium, atomic number 2, remains the even-numbered counterpart to hydrogen.
Thus, neutral hydrogen—or hydrogen paired with an electron, the only stable lepton—constituted the vast majority of the remaining unannihilated portions of matter following the conclusion of inflation.
Another exception to the rule is beryllium, which, despite an even atomic number (4), is rarer than adjacent elements (lithium and boron).
This is because most of the universe's lithium, beryllium, and boron are made by cosmic ray spallation, not ordinary stellar nucleosynthesis, and beryllium has only one stable isotope (even that is a Borromean nucleus near the boundary of stability), causing it to lag in abundance with regard to its neighbors, each of which has two stable isotopes.
) 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.
"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.
[8] 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.
Each of the light elements oxygen, neon, magnesium, silicon, and sulfur, have two isotopes with even isospin (nucleon) parity.
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.
[citation needed] Depending on the mass of a star, the Oddo–Harkins pattern arises from the burning of progressively more massive elements within a collapsing dying star by fusion processes such as the proton–proton chain, the CNO cycle, and the triple-alpha process.
The newly formed elements are ejected slowly as stellar wind or in the explosion of a supernova and eventually join the rest of the galaxy's interstellar medium.
