Neutronium (Nu) is an element.
Neutronium was proposed by scientist Andreas von Antroproff in 1926. He placed it above helium and gave it the symbol -.This would be changed to Nu. The neutron was later discovered in 1932. Neutronium is not considered by all as an element. If it is, there are many isotopes.
Neutronium has 4 major known isotopes:
Nu-0, which has no neutrons, and is stable
Nu-1, which has one neutron, and beta decays to Hydrogen-1 in 10 minutes
Nu-2, which has 2 neutrons, and emits a neutron to Neutronium-1 in an unknown amount of time
Nu-4, which has 4 neutrons, and emits 3 neutrons to Neutronium-1 or 2 neutrons to Neutronium-2 in 10^-21 seconds
Neutronium in neutron stars can act like high isotopes of neutronium. The mass of an atom of neutronium is 1.67×10^−27 kg and the mass of a neutron star goes from 1.1 to 2.14 SM. Therefore, if they are accepted, the "neutron star isotopes" go from Nu-1E57 to Nu-3E57.
Neutronium has no electronic structure (having no electrons) and as such cannot form bonds with other elements. This is why it is placed above helium. Neutronium is a gas at all temperatures and pressures, but becomes superfluid at extremely high pressures.