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Neutronium
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<p>Neutronium (or neutrium, neutrite, or element zero) is a hypothetical substance made purely of <a href="/facts/Neutron/oRCM1geb">neutrons</a>. The word was coined by scientist <a href="/facts/Andreas_von_Antropoff/ktx8ceCn">Andreas von Antropoff</a> in 1926 (before the 1932 <a href="/facts/Discovery_of_the_neutron/WbqBAp6G">discovery of the neutron</a>) for the hypothetical "element of atomic number zero" (with no protons in its nucleus) that he placed at the head of the <a href="/facts/Periodic_table/umdGIfUb">periodic table</a> (denoted by -). However, the meaning of the term has <a href="/facts/Semantic_change/aYJrWBot">changed over time</a>, and from the last half of the 20th century onward it has been also used to refer to extremely dense substances resembling the <a href="/facts/Neutron-degenerate_matter/S5bc5MwK">neutron-degenerate matter</a> theorized to exist in the cores of <a href="/facts/Neutron_star/4enfoxBA">neutron stars</a>. </p>