Roentgenium-282
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Roentgenium (111Rg) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was 272Rg in 1994, which is also the only directly synthesized isotope, all others are decay products of nihonium, moscovium, and tennessine. There are 7 known radioisotopes from 272Rg to 282Rg. The longest-lived isotope is 282Rg with a half-life of 2.1 minutes, although the unconfirmed 286Rg may have a longer half-life of about 11 minutes.
Super-heavy elements such as roentgenium are produced by bombarding lighter elements in particle accelerators that induce fusion reactions. Whereas the lightest isotope of roentgenium, roentgenium-272, can be synthesized directly this way, all the heavier roentgenium isotopes have only been observed as decay products of elements with higher atomic numbers.
Depending on the energies involved, fusion reactions can be categorized as "hot" or "cold". In hot fusion reactions, very light, high-energy projectiles are accelerated toward very heavy targets (actinides), giving rise to compound nuclei at high excitation energy (~40–50 MeV) that may either fission or evaporate several (3 to 5) neutrons. In cold fusion reactions, the produced fused nuclei have a relatively low excitation energy (~10–20 MeV), which decreases the probability that these products will undergo fission reactions. As the fused nuclei cool to the ground state, they require emission of only one or two neutrons, and thus, allows for the generation of more neutron-rich products. The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (see cold fusion).
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