Unbihexium atomic number 126
Excellent this did not take as long as I expected. Atomic number 126, known formally as unbihexium, is the proposed atomic number of Kryptonite. This element is a favorite of the nuclear physicist/Superman fanboy geek subset—a bigger demographic than you would expect—because unbihexium is the heaviest theoretical double-magic transuranic on the nuclear island of stability.
Physicist Glenn Seaborg, one of a select few to have an element named in his honor during his lifetime—seaborgium, atomic number 106—first proposed the so-called island of stability on the upper fringes of the periodic table. The island is a range of superheavy elements that, despite their three-figure atomic numbers, would prove comparatively stable, with theoretical half-lives measuring longer than a few seconds.
The idea behind the island is that the atomic nucleus' organization occurs by shells, just like electron levels. Each shell could accommodate a certain number of protons or neutrons, and a "complete" shell would produce a stable element.
As atomic numbers get larger, each subsequent outer shell requires more protons or neutrons to be complete, so heavier stable elements are rare. The island of stability theory holds that certain "magic numbers" of protons or neutrons would produce stable shells: 114, 120, or 126 protons, and 184 neutrons.
Therefore, the isotope unbihexium-310 (126 protons and 184 neutrons) would be "double magic." As the heaviest theoretically stable atomic isotope, unbihexium is a favorite of science-fiction writers—and the most often-cited candidate for a true-life Kryptonite.
