Iso-8 stands for Isotope 8. So that would mean an atom with a molecular weight of 8. And since there’s an unstable version of Iso-8, that would mean there’s another material with an 8 Isotope but which has a half-life.
It can’t be any higher than Oxygen, since that’s got 8 protons. It’s also shown in a crystalline form at normal temperatures, so it can’t be any of the gasses like Hydrogen (H), Helium (He), Nitrogen (N), or Oxygen (O). Which means that the only options left are Lithium (Li, 3 protons, Iso-7/6), Beryllium (Be, 4 protons, Iso-9), Boron (B, 5 protons, Iso-11/10), and Carbon (C, 6 protons, Iso-12/14)
Carbon-8 does technically exist, but having 6 protons and only 2 neutrons is an inherently unstable nuclear make up. Carbon-8 is highly radioactive (completely disintegrates through alpha and beta radiation) and has a half-life of less than 0.00000000000000000000199 seconds (that’s 21 zeroes). So Carbon-8 is out as both stable and unstable Iso-8.
Beryllium-8 would seem a logical conclusion with it’s even distribution of 4 protons and 4 neutrons. Beryllium is already rare in it’s regular 9 isotope, but the Iso-8 variant is so unstable (decaying with a half-life of 0.00000000000000007 seconds into 2 Helium atoms) that it would be gone before you would even have a chance to pick it up. So Beryllium is also out.
Boron-8 is an unstable isotope with a half-life of 0.770 seconds. That is stable enough to exist, but likely not in great quantities. Boron can also only be found in chemically pure form in meteorites. So it’s likely that this is the Unstable Iso-8 substance.
Which leaves Lithium-8 as the only possibility for “regular” Iso-8. With a half-life of 0.868 seconds it is slightly more stable than Boron-8, which would justify the name Unstable Iso-8 for Boron-8, but the difference is negligible. Lithium has widespread uses in nuclear applications (as in the production of Tritium or Hydrogen-3), electronics (the common Lithium Ion batteries), and medical applications (treating bipolar disorder). In the context of Iso-8 in the Marvel Puzzle Quest story-lines, Lithium fits best, cuz let’s face it, who’s more bipolar than The Hulk? However, what exactly the benefit would be of having an 8 isotope in particular, I have no idea.
TL;DR
So my conclusion:
Iso-8 = Lithium-8
Unstable Iso-8 = Boron-8
