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In the beginning, there was a shitload of energy.


Eventually (after fractions of a second), the universe cooled off enough that small subatomic particles, such as quarks, could form. Later, it cooled off enough that protons, neutrons, and electrons, the basic building blocks of atoms, could exist.


These nucleons combined to make the atoms we all know and love. Hydrogen and its varieties; protium, deuterium, tritium. Helium-3 and helium-4. But at that point, everything just about stops. Why?


As it turns out, there are no stable atoms with a mass of 5. Sure, you can smash together bits in a lab to get atoms with mass five. Helium-5 (2 protons, 3 neutrons) decays in 10-24 seconds, many orders of magnitude quicker than even exotic superheavy transuranic isotopes. Lithium-5 disintegrates almost as fast. (To my knowledge, beryllium-5 and boron-5 do not exist, and if they did we could assume they would likewise be unstable). The hyperphysics site has a bit about it here; http://hyperphysics.phy-astr.gsu.edu/hbase/astro/mass5w.html


But why is helium-5 so unstable? Tritium has twice as many neutrons as neutrons, and while it's unstable, it still has a half life measured in years (about 12 of them). If you counted one bacterium every half life of helium-5, you could count all the bacteria on Earth a hundred times over in tritium's half life (thanks wikiped). This is despite the fact that helium-5's neutron-proton ratio is 1.5/1; there are numerous nucleons with a ratio greater than this, even notoriously stable lead-208. (Lithium-5's instability is not surprising, as a nucleon with more protons than neutrons. Although carbon-10 with the same proton-neutron ratio has a half life measurable in whole seconds.) Nuclei with even protons and odd neutrons are common, odd-odd pairings are usually more unstable (unless you're tantalum-180m). In short, emot-iiam.gif (I'm sure there is a good reason, but I'm both not a trained nuclear physicist and mildly intoxicated right now.)


But what does this all mean? As the early universe cooled, atoms were able to fuse together into heavier elements. But the lack of a stable mass-5 isotope stopped this. A few helium and hydrogen nuclei were able to make lithium and beryllium isotopes with mass 6 and 7, but the lack of a mass five intermediary meant that this could only happen through collisions of existing heavy atoms (rather than adding a single neutron or proton to another atom). The lack of a stable mass 8 isotope put a firm stop to the process. As a result, the vast majority of the universe end up as hydrogen and helium. As it turned out, this is the perfect fuel for stars. Red dwarfs, the most common type of start, are too small to fuse lithium, beryllium, boron, and other heavy elements. If helium-5 or lithium-5 were stable, the universe would be completely unrecognizable.



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The theoretical reason for the stability of even-even/instability of odd-odd nucleons is that the strong force (responsible for binding nuclei) is stronger if the nucleons can form pairs with aligned spin angular momentum. When you have odd-odd nucleons, you essentially have an extra set of nucleons, and the strong force is not enough to hold them together under most circumstances.

This is based on what I remember from a nuclear/particle physics class 5 years ago.

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