Neutronium

Neutronium (or neutrium,[1] neutrite,[2] or element zero) is a hypothetical substance made purely of neutrons.

The word was coined by scientist Andreas von Antropoff in 1926 (before the 1932 discovery of the neutron) for the hypothetical "element of atomic number zero" (with no protons in its nucleus) that he placed at the head of the periodic table (denoted by -).

[3][4] However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been also used to refer to extremely dense substances resembling the neutron-degenerate matter theorized to exist in the cores of neutron stars.

[6] The term "neutronium" was coined in 1926 by Andreas von Antropoff for a conjectured form of matter made up of neutrons with no protons or electrons, which he placed as the chemical element of atomic number zero at the head of his new version of the periodic table.

[3] It was subsequently placed in the middle of several spiral representations of the periodic system for classifying the chemical elements, such as those of Charles Janet (1928), Edgar Emerson (1944),[7][8] and John D. Clark (1950).