The element that has the lowest mass per nuclear particle and therefore cannot release energy by either fusion or fission is the element Fe or iron. Iron is referred to as a special element due to its nuclear binding energy. A general idea is that you can have nuclear fusion of two elements and get a heavier element only up to the element iron in the periodic table of elements and also you can only have fusion reactions starting from the element next to iron. This is due to the nuclear forces and the electromagnetic force being balanced for the element iron.