The Oddo-Harkins Rule shows the universe hates the odd

When scientists started discovering the elements, they noticed a strange pattern. Today, that pattern is known as the Oddo-Harkins Rule. And while we understand some of the reason for the rule, there's still a mystery about it.

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As the elements acquired protons, they became more and more rare, but certain elements were far more rare than others. The even-numbered elements, with their even numbers of protons, were far more abundant than the odd-numbered elements. Graphs of the abundance of elements in the universe resembled toothed saws, with the even numbered elements making the teeth, and the odd numbered elements making the recesses between them.


The two people who first discovered this, Giuseppe Oddo and William Draper Harkins, lent their names to the Oddo-Harkins Rule. An even-numbered element will, in general, be more abundant than the odd-numbered elements to either side of it.

Why does the universe so hate the oddballs? Partly because of how elements are made. Stars are giant balls of hydrogen gas that ignite when the gravitational pressure from all that gas causes the hydrogen at the center to fuse into helium. This continues until the hydrogen is pretty much used up, and only the helium remains. Helium has an atomic number of two, and so when it combines, it will yield another even numbered element. When that element is combined, it will yield another even numbered element. Basically, once you go even, you never go back. The odd numbered elements have to be made with the leftover wisps of hydrogen or in the final death throes of a star, when atoms capture wandering neutrons, which then degrade into protons.


There's also the fact that some elements degrade a lot more slowly than others, especially with the ones with a ratio of two neutrons for every proton. When few odd elements are built, and many of them degrade pretty fast, we're stuck in an even, even universe.


Image: ESO

[Via Doklady Earth Sciences, Oxford University Press, General Chemistry, The Origin of the Earth]


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