A neutron is an atomic nucleus with 0 electrons. The same is true for hydrogen ionized once, helium ionized twice, lithium ionized thrice, and so on. From the

A neutron is an atomic nucleus with 0 electrons. The same is true for hydrogen ionized once, helium ionized twice, lithium ionized thrice, and so on. From the point of view of chemistry, a neutron is not more subatomic than a hydrogen nucleus or any other nucleus. There is indeed a difference between the nuclei with A = 1 and the nuclei with A > 1, because the latter can be decomposed into the former at high enough temperatures, of a few tens of MeV, similarly to the decomposition of any multi-atomic molecule at high enough temperatures. "Nothing" is not an isotope of neutron, for the same reason why it is not an isotope of sulfur or of iron, i.e. the mass number A is defined as a positive integer number and there is no reason to define it otherwise. On the other hand, the atomic number Z can be defined as an integer number if antimatter is also considered, or as a non-negative integer when applicable only to ordinary matter. Considering neutron as a noble gas like the others simplifies certain descriptions by eliminating special cases that must be considered separately. The noble gases can usually be ignored in chemistry, because they typically do not participate in chemical reactions, but in nuclear reactions there is nothing special about them, so the noble gases, including the neutron, participate in nuclear reactions like any other elements.