Having an even number of neutrons and/or an even number of protons tends to make a nucleus more stable against radioactive decay:
• Wikipedia, Even and odd nuclei.
I just learned there are only 5 stable nuclei with an odd number of neutrons and an odd number of protons:
• deuterium (hydrogen-2), with 1 proton and 1 neutron.
• lithium-6, with 3 protons and 3 neutrons.
• boron-10, with 5 protons and 5 neutrons.
• nitrogen-14, with 7 protons and 7 neutrons.
• tantalum-180, with 73 protons and 107 neutrons.
Deuterium is rare compared to hydrogen and helium-4. Lithium-6 is rare compared to lithium-7.
Tantalum-180 is rare compared to tantalum-181, though I was lying slightly when I said it was stable: theoretically it’s predicted to decay, though with such a long half-life—over 1016 years—that it’s never actually been seen to decay. It’s also weird because it’s the only nuclear isomer found naturally in nature: that is, a nucleus that’s in an excited state, not its ground state. To add to the weirdness, the ground state of tantalum-180 is less stable than the excited state: it decays into tungsten or hafnium with a half-life of 8 hours!
Anyway: in these cases, one gets the feeling that odd-odd nuclei are hard for nature to produce. But 20% of boron is boron-10 (the rest being boron-11), and 99.6% of nitrogen is nitrogen-14 (the rest being nitrogen-15).
What’s up with nitrogen-14? Why is it the most abundant isotope of an element despite it being doubly odd? Or more precisely, why is it the only isotope with this property?
nuclei —-> neutrons
Fixed — thanks!
Keith McClary writes:
Primary nitrogen is nitrogen formed before stars using the CNO cycle:
PM 2Ring writes:
From the same Wikipedia article:
Thanks. Somehow I missed that! Of course, the CNO cycle destroys as much nitrogen-14 as it creates!
But I guess if something creates either carbon-12 or oxygen-16, the CNO cycle will turn some of that into nitrogen-14.
This is vaguely connected to my next post:
• The Hoyle state.
which concerns how carbon-12 gets produced in the first place.
I guess carbon-12 gets produced from collisions between beryllium-8 and helium-4, and then some of it becomes nitrogen-14.
I should know this but I don’t. If the cycle is complete, then, yes, nitrogen is essentially a catalyst and neither created nor destroyed. But perhaps it is not always complete?
I’m sure that someone knows the answer to this!
When a star gets old and starts emitting lots of gas (a “planetary nebula”) or blows up (a “nova” or “supernova”), presumably some nitrogen still hanging around gets out.
The CNO cycle must be sort of like a carousel in a fairground: when it stops going round and round, you get off wherever you are.
Could it do with the # of protons + neutrons being half of the magic number to fill a nuclear shell? E.g., “These numbers of protons or neutrons (2, 8, 20, 28, 50, 82, and 126) make complete shells in the nucleus”, and I’ll note that Boron is half of 20 and Nitrogen is half of 28, and those are your two exceptions.
Alternatively, is the N15 capture cross-section just larger than the N14 capture cross section, so by kinetics you end up with a lot of N14 and little N15. (but that raises the why? question, which takes me back to maybe half of magic numbers)
Half magic! Hmm, that’s an interesting idea. One problem is that nuclei are especially stable when either the number of protons or the number of neutrons — or both — is a magic number. The 7 protons and 7 neutrons in nitrogen-14 are each a quarter of the magic number 28. I don’t even know if something nice happens when the sum of the number of protons and neutrons is a magic number, much less half a magic number.
Deuterium is half helium-4, which in some ways deserves to be called an independent particle in its own right, in, for example, alpha decay.
A wild speculation, but perhaps fun:
Picture:
In the Fano plane, there are 7 “points”-> Protons, and 7 lines ->Neutrons. You can create a 3D version of the Fano plane in the shape of an octahedron: The points are the 6 vertices plus 1 center point. The 7 lines are 4 alternate faces of the octahedron plus 3 axis. (XYZ).
Each proton is then surrounded by 3 neutrons, and each neutron is surrounded by 3 protons.
The strong force (as I just learned from Wikipedia), attracts best between adjacent protons and neutrons with the same spin. But proton-proton and neutron-neutron with the same spin don’t like being adjacent because of Pauli. So when the numbers are even, it is easier to form pairs of the same spin, while keeping the total spin zero.
Over on Physics StackExchange, ProfRob writes: