 Superheavy elements are produced by nuclear fusion . These fusion reactions can be divided into " hot " and " cold " fusion , depending on the excitation energy of the compound nucleus produced . 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 ( which use heavier projectiles , typically from the fourth period , and lighter targets , usually lead and bismuth ) , 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 . Hot fusion reactions tend to produce more neutron @-@ rich products because the actinides have the highest neutron @-@ to @-@ proton ratios of any elements that can presently be made in macroscopic quantities . 
