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?