The Oddo-Harkins rule states that chemical elements with even atomic numbers are more abundant than the adjacent odd atomic number elements. For example, oxygen (atomic number 8) is more abundant than either nitrogen (atomic number 7) or fluorine (atomic number 9). Calcium (20) is more abundant than potassium (19) or scandium (21). Credit for discovering this pattern goes to Guiseppe Oddo in 1914 and William Draper Harkins in 1917.
Explanation of the Oddo-Harkins Rule
Atoms form when protons and neutrons bind together and form an atomic nucleus. For most elements, this happens when the immense temperature, pressure, and gravity within a star fuses protons and neutrons together. An element’s atomic number is the number of protons in its atom.
One explanation for the higher abundance of even-numbered elements is that helium (atomic number 2) is a major building block for element formation. Fusion of helium nuclei builds subsequent even atomic number elements. Another explanation is that even atomic numbers mean protons are paired within the nucleus. Parity makes the nucleons more stable, as the spin of one proton offsets the spin of the other. Unpaired protons (odd number elements) more easily capture another proton and form an even-numbered atom.
Chemists and astronomers see the Oddo-Harkins rule in action when massive stars die and explode. While different stars uphold the rule, their element ratio differs according to the metallicity of the star.
Exceptions to the Oddo-Harkins Rule
Elements that serve as two exceptions to the Oddo-Harkins rule are hydrogen (atomic number 1) and beryllium (atomic number 4). Hydrogen is the most abundant element in the universe. It is more abundant than helium because of its creation in the Big Bang. However, stars continuously fuse hydrogen into helium. In the distant future, hydrogen will follow the even-odd rule.
Beryllium is even more rare than lithium (atomic number 3) and boron (atomic number 5), even though the primary source of the all three elements is cosmic ray spallation. Scientists believe beryllium does not follow the rule because it only has one stable isotope. Lithium and boron each have two stable isotopes.
References
- Galarza, Jhon Yana; Meléndez, Jorge; et al. (2021). “Explosive nucleosynthesis of a metal-deficient star as the source of a distinct odd-even effect in the solar twin HIP 11915”. Monthly Notices of the Royal Astronomical Society: Letters. 502 (1): L104-L109. doi:10.1093/mnrasl/slab010
- Harkins, William D. (1917). “The Evolution of the Elements and the Stability of Complex Atoms”. Journal of the American Chemical Society. 39 (5): 856–879. doi:10.1021/ja02250a002
- North, John (2008). Cosmos – An Illustrated History of Astronomy and Cosmology. Univ. of Chicago Press. ISBN 978-0-226-59441-5.
- Oddo, Giuseppe (1914). “Die Molekularstruktur der radioaktiven Atome”. Zeitschrift für Anorganische Chemie. 87: 253–268. doi:10.1002/zaac.19140870118