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Neutronium is a term originally used in science fiction and in popular literature to refer to an extremely dense phase of matter composed primarily of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 (scil., before the discovery of the neutron itself) for the conjectured 'element of atomic number zero' that he placed at the head of the periodic table.^[1][2] However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been used legitimately to refer to extremely dense phases of matter resembling the neutron-degenerate matter postulated to exist in the cores of neutron stars.

Main article: neutron star

The term neutronium is used in popular literature to refer to the material present in the cores of neutron stars (stars which are too massive to be supported by electron degeneracy pressure and which collapse into a denser phase of matter). This term is very rarely used in scientific literature, for two reasons:

• There is no universally agreed-upon definition for the term "neutronium".
• There is considerable uncertainty over the composition of the material in the cores of neutron stars (it could be neutron-degenerate matter, strange matter, quark matter, or a variant or combination of the above).

When neutron star core material is presumed to consist mostly of free neutrons, it is typically referred to as neutron-degenerate matter in scientific literature.

The term neutronium was coined in 1926 by Professor Andreas von Antropoff for a conjectured form of matter made up of neutrons with no protons, which he placed as the chemical element of atomic number zero at the head of his new version of the periodic table. It was subsequently placed as a noble gas in the middle of several spiral representations of the periodic system for classifying the chemical elements, such as the Chemical Galaxy (2005). It is thought to be the first element that emerged after the big bang.

Although the term is not used in the scientific literature either for a condensed form of matter, or as an element, there have been reports that, besides the free neutron, there may exist two bound forms of neutrons without protons. However, these reports have not been further substantiated. Further information can be found in the following articles:

• Unineutron: Isolated neutrons undergo beta decay with a half-life of approximately 10 minutes, becoming protons (the nucleus of hydrogen) and electrons.
• Dineutron: The dineutron, containing two neutrons, is not a bound particle, but has been proposed as an extremely short-lived state produced by nuclear reactions involving tritium.
• Tetraneutron: A tetraneutron is a hypothetical particle consisting of four bound neutrons. Reports of its existence have not been replicated. If confirmed, it would require revision of current nuclear models.

A trineutron state consisting of three bound neutrons has not been detected, and is not expected to be stable even for a short time. Calculations indicate that the hypothetical pentaneutron state, consisting of a cluster of five neutrons, would not be bound.

And so on, through the numbers, up to icosneutron, with 20 neutrons.^[3]

• Neutron star
• Degenerate matter
• Compact star