The atomic number 26(iron) is the threshold value below which the fusion might occur.
Explanation:
Nuclear fusion is a reaction in which two or more nuclei are combined to form one or more different atomic nuclei and subatomic particles.
Energy released in a fusion reaction is because of a key feature of nuclear matter called the binding energy which is a measure of the efficiency with which its constituent nucleons are bound together.
As we go up in atomic number, the energy released per nuclei goes down until it hits a minimum which is for atomic number 26 (iron) and fusion is not possible.
Nuclear Fusion is a process in which two or more nucleus are fused(combined) together to make heavier nucleus and release a lot of energy and other subatomic particles in the process. Considerable amount of energy is required to fuse the nucleus together but the output energy is much more than the input energy. It becomes an exothermic process.
In case of heavier nuclei, this doesn't hold true as the process becomes endothermic. That means the output energy will be way lesser than the input energy given to fuse the nucleus. This happens for elements of atomic no. 26 or above. Even in stars we can find element till atomic no. 26 as they can be formed by fusion. When a star reaches to iron fusion stage that is the end of it and it explodes into a supernova.
