EDIT: Thanks for all the responses!
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Oganesson, element 118, is just where we’ve got up to *so far*. It makes the periodic table “look finished” but that’s just coincidence right now and we are trying to make element 119 and beyond.
Part of the problem is that the more protons a nucleus has, the more neutrons per proton it needs for maximum stability. So when we fuse two smaller nuclei to get one with lots of protons, there’s not really enough neutrons in the product. For everything from Bohrium (element 107) onwards, the most stable isotope we know is the one with the most neutrons. They might have more stable isotopes we haven’t discovered yet – although “more stable” for the superheavy elements is likely to mean half lives of a few hours at most.
Making it even harder is to get up to Oganesson we bombarded target isotopes with Calcium-48 – 20 protons, 28 neutrons. That helped with the neutrons per proton issue. Even then, it’s rare that the fusion we want actually happens. But to make element 119 that way we’d need a target of element 99, Einsteinium, and we haven’t made enough of *that*. So we either need to make a lot more Einsteinium or use a different isotope for the “projectile”, and the second choice is what researchers are trying, but projectiles with more protons aren’t as good on the neutron:proton ratio.
It is reckoned that at some number of protons, no isotopes will be stable enough to form atoms, but theories don’t agree on what that number is. It takes a few femtoseconds for the electrons to gather around the nucleus so that’s what imposes the minimum half life to call something an “element”. Some theories also find that at some number of protons the atom can’t hold onto all its electrons because the outer ones would need to travel faster than light, but again, theories don’t agree on the details.
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