Heaviest element to produce a net release of energy via nuclear fusion

All of the elements lighter than iron, which has an atomic number of 26, are created by nuclear fusion in the cores of stars. Called stellar nucleosynthesis, this process creates heavier elements by the fusing of two lighter elements, which also releases vast amounts of energy. All of the elements heavier than iron require energy to be added in order to fuse into heavier elements. As a result, once a star has produced iron, its energy output decreases dramatically and the star collapses. The shockwave produced when the collapsing star meets its own incompressible core causes further nuclear fusion that creates all of the naturally occurring heavier elements that are then dispersed into space in a supernova.

The theory of stellar nucleosynthesis was first developed by English astronomer Sir Fred Hoyle in 1954.