Fermium

Fermium marks a hard wall. Every element up to it can be reached by neutron capture followed by beta decay, which is how reactors and thermonuclear explosions make actinides in bulk. But fermium-258 fissions spontaneously with a half-life of 370 microseconds, so the chain cannot climb past mass 257 — the so-called fermium gap — and everything heavier must be built by slamming two nuclei together in an accelerator, one atom at a time. That is why the character of element-making changes so completely at 101. Fermium was found alongside einsteinium in the Ivy Mike debris and named for Enrico Fermi, who built the first reactor; the largest sample ever produced weighed a few hundred picograms.