Oddo–Harkins rule
The Oddo–Harkins rule holds that an element with an even atomic number is more abundant than the elements with immediately adjacent atomic numbers. For example, carbon, with atomic number 6, is more abundant than boron (5) and nitrogen (7). 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. This pattern was first reported by Giuseppe Oddo[1] in 1914 and William Draper Harkins[2] in 1917.[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]
Definitions
All
The rule
The early form of the rule derived from Harkin's 1917 study of meteorites. He reasoned as others at the time, that meteorites are more representative of the cosmological abundance of the elements. Harkins observed that even atomic number (Z) elements were about 70 times more abundant than those with odd Z. The first seven elements, making up almost 99% of the material in a meteorite, were all even numbered Z. In addition, he observed that 90% of the material consisted of only 15 different isotopes, with atomic weights in multiples of four, the approximate weight of
Relation to stellar nucleosynthesis
The Oddo-Harkins rule for elements from 12C to 56Fe is explained by the
Exceptions to the rule
This postulate, however, does not apply to the universe's most abundant and simplest element:
This period allowed for the fusion of single protons and deuterium nuclei to form helium and lithium nuclei but was too short for every H+ ion to be reconstituted into heavier elements. 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, causing it to lag in abundance with regard to its neighbors, each of which has two stable isotopes.
Isotopic abundance
The elemental basis of the Oddo-Harkins has direct roots in the isotopic compositions of the elements.[7] 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.[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. 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.
Relationship to fusion
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
See also
- Abundance of elements in Earth's crust
- List of elements by stability of isotopes
- Nuclear chemistry – Branch of chemistry dealing with radioactivity, transmutation and other nuclear processes
References
- .
- .
- ISBN 978-0-226-59441-5.
- ISSN 0034-6861.
- ^ .
- ISBN 978-1-4020-5544-7.
- ^ ISSN 0047-2689.
- ^ "The Nobel Prize in Physics 1963". NobelPrize.org. Retrieved 2024-02-01.