Neutronium or Element Zero

Neutronium or Element 0
Neutronium is the hypothetical element zero, with no protons in its atomic nucleus.

Neutronium is the name of a theoretical element with atomic number 0 and symbol Nu that consists entirely of neutrons. Other names for neutronium are neutrium and neutrite. Chemist Andreas von Antropoff coined the term “neutronium” in 1926 (before the discovery of the neutron).

Whether or not neutronium is an element depends on your definition of the term. Most chemists define elements based on the number of protons in the atomic nucleus. Chemical reactions involve electrons, which neutronium lacks. At the same times, nuclear reactions allow neutrons to become protons (hydrogen).

Location of Neutronium on the Periodic Table

Neutrons don’t have a net electrical charge, so they don’t attract electrons. Therefore, as far as electron configuration goes, neutronium behaves as a noble gas. This puts neutronium above helium on the periodic table, in period 0 and element group 18.

Neutronium Facts

Name: Neutronium

Atomic Number: 0

Group: 18 (noble gas)

Period: 0

Electrons per Shell: 0

Discoverer: Not discovered, but suggested by Andreas von Antropoff (1928)

Atomic Weight: 1 (predicted)

Neutronium Isotopes

The isotopes of an element depend on the number of neutrons in its nucleus. As of 2021, two neutronium isotopes have been definitely observed (mononeutron and dineutron), while others have been proposed. These “isotopes” have simple, descriptive names:

  • Mononeutron: This is a single neutron, which has a half-life of about 10 minutes and decays via beta decay into a proton (a hydrogen nucleus), an electron, and an antineutrino.
  • Dineutron: A decay of beryllium-16 in 2012 resulted in the release of a dineutron, which consists of two neutrons. These neutrons aren’t bound together like protons and neutrons in a nucleus, but are attracted to each other sufficiently that they are comparable to a nucleus. Researchers propose dineutrons may also occur in short-lived tritium resonance states.
  • Trineutron: Trineutrons would consist of three bound neutrons, but the system would be so thermodynamically unfavorable that it’s unlikely trineutrons exist.
  • Tetraneutron: A tetraneutron consists of four bound neutrons. Early research indicates tetraneutrons may exist, but results have not been replicated. Alternatively, an observed tetraneutron might be a pair of bound dineutrons, forming a molecule rather than an isotope.
  • Pentaneutron: A pentaneutron consists of five bound neutrons. Scientists have calculated the potential stability of a five neutron cluster.

Neutronium Properties

Although bulk neutron matter has not been observed, scientists are able to make predictions about its properties:

  • Neutronium would be chemically inert, like a noble gas. This is because electrons are the key players in chemical reactions. Neutrons neither attract nor bind electrons.
  • Since neutronium gas lacks electrons to scatter light, it would presumably be colorless. The appearance of a hypothetical solid is anyone’s guess.
  • Because of the low particle attraction, neutronium would be a gas. The ideal gas laws predicts neutronium density at standard temperature and pressure as 0.045 kg/m3, which is half as dense as hydrogen gas. Near absolute zero, neutronium could form a degenerate gaseous superfluid. Solidification might occur under extreme pressure, which might also inhibit beta decay and make neutronium stable.
  • Neutronium gas should be more compressible than ordinary gases, due to the lack of electrons and protons. Electron shells account for most of an atom’s volume, while protons repel one another in close proximity.
  • Keeping neutronium in a container would be problematic, as neutrons are small enough to pass between atoms and molecules. The effect would be comparable to putting helium gas in a latex balloon. Similarly, neutronium can’t be trapped using electric fields because it doesn’t have a net charge.

In Fiction

Neutronium appears in science fiction books, movies, and video games where it is a form of incredibly dense and usually solid matter. In the mid-20th century, neutronium was featured in the Star Trek episode “Doomsday Machine” and H. Beam Piper’s “Terrohuman Future History” novels (as collapsium). It’s an element in the video game “Oxygen Not Included.”

Neutronium and Neutron Stars

In the context of physics, “neutronium” most often refers to neutron star matter. However, in scientific literature the preferred term is “neutron-degenerate matter.” Other researchers postulate neutron stars contain strange matter or quark matter. In any case, referring to all the matter of a neutron star as “neutronium” is incorrect because the composition of stars varies depending on depth and pressure.


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  • Bevelacqua, J. J. (June 11, 1981). “Particle stability of the pentaneutron”. Physics Letters B 102 (2–3): 79–80. doi:10.1016/0370-2693(81)91033-9
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