When a neutron star explodes, its gravity weaken so neutronium may fission, is there a mass defect as lighter elements than neutronium form? As we get towards iron energy is released? Or not?
1 Answer
There are a number of misconceptions in your question.
Neutronium does not exist. Its name was coined in 1926 to designate a hypothetical element number zero (no protons in the nucleus). The core of a neutron star is sometimes said to consist of neutronium, but mostly in popular literature. In reality, a neutron star core is effectively one gigantic atomic nucleus.
Neutron stars do not explode. However, they can implode into a black hole. That does not create any lighter elements. If there was an "element zero" then no lighter element could exist anyway.
Iron is the element with the highest mass defect. As more protons and neutrons are added to light nuclei, energy is released in the merge (nuclear fusion) - up until iron is reached. Beyond iron, iron is released as the nucleus is split (nuclear fission).
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1$\begingroup$ Corrections: iron and nickel have the largest binding energy per nucleon. Heavier elements have larger total binding energy. Also, while an isolated neutron star cannot explode, neutron star mergers are violent events responsible for heavy element production. $\endgroup$– rob ♦Commented Jan 31, 2022 at 17:54