Elements with atomic number 26, specifically iron (Fe), are not formed by nuclear fusion in the cores of very massive stars because fusion reactions that involve iron nuclei do not release energy; instead, they require an input of energy. This phenomenon is known as the iron peak.
During stellar nucleosynthesis, fusion reactions occur in the cores of stars, fusing lighter elements into heavier ones and releasing energy in the process. This process continues until the core reaches the fusion of silicon (Si) into nickel (Ni). At this point, the fusion reactions that lead to the formation of iron nuclei become energetically unfavorable.
Iron has the highest binding energy per nucleon (the energy required to separate a nucleus into its individual protons and neutrons) among all the elements, which means that fusing iron nuclei or breaking them apart requires energy rather than releasing it. As a result, the fusion of lighter elements into heavier ones ceases at iron, and the core of the star cannot sustain nuclear reactions.
When a massive star reaches this stage, it has exhausted its nuclear fuel and has no energy source to counteract the gravitational forces pushing inward. The core collapses under its own gravity, leading to a supernova explosion. During the supernova event, elements heavier than iron, such as gold, platinum, and uranium, can be synthesized through rapid neutron capture processes (r-process) that occur in the energetic environment of the explosion.
In summary, the inability of fusion reactions involving iron nuclei to release energy is the reason why elements with atomic number 26 are not formed by nuclear fusion in the cores of very massive stars.