Hydrogen-5

Hydrogen-5 is a highly unstable isotope of hydrogen. The nucleus consists of a proton and four neutrons. It has been synthesised in the laboratory by bombarding tritium with fast-moving tritium nuclei (see this article). In this experiment, the one tritium nucleus captures two neutrons from the other, becoming a nucleus with one proton and four neutrons. The remaining proton may be detected, and the existence of hydrogen-5 deduced. It decays through neutron emission and has a half-life of 8.01930x10^-23 seconds.